JP2021041376A - Coating method for automobile body - Google Patents

Abstract

To provide a coating method that obtains a high-design coating film that is high in color saturation and excellent in brilliance and feeling of depth and that is able to inhibit a color change caused by a film-thickness change.SOLUTION: In a coating method for an automobile body including a step of forming a first multi-layer coating film on an external plate portion of the automobile body, the absolute value of the difference between the L*value of an intermediate coating film and the L*value of a first base coating film on the external plate portion is equal to or smaller than 35, first and second base coating materials contain no scale-like aluminum pigment, the absolute value of the difference between the hue angle of the first base coating film on the external plate portion and the hue angle of a second base coating film is equal to or smaller than 16, the second base coating material contains scale-like mica pigment and coloring pigment, the content of the scale-like mica pigment is 0.1% or more by mass and 3% or less by mass of the second base coating material based on a solid content, the light transmittance, at a wavelength of 400 to 700 nm, of the second base coating material is 40% or more and 60% or less, and the hardened film thickness of the second base coating film is 4-12 μm.SELECTED DRAWING: None

Description

The present invention relates to a method for painting an automobile body.

The main purpose of painting the car body is to protect the car body and give it an aesthetic appearance. Specific examples of vehicle body protection include protection from the natural environment such as ultraviolet rays, water, and heat (weather resistance), suppression of rust generation (corrosion resistance), and scratch prevention (scratch resistance).

On the other hand, aesthetics appeal to human sensibilities and sensations, and are an important factor in selecting a car. For this reason, there is an increasing need for a highly-designed coating film that gives a good aesthetic appearance to an automobile body year by year.

As a method for obtaining such a highly designed coating film, for example, in Patent Document 1, a metallic base coating film containing a coloring pigment and a bright material is applied to the surface of an object to be coated to obtain a metallic base coating film. Step (1), a step (2) of forming a coloring base coating film by applying a coloring base coating film containing a coloring pigment on the metallic base coating film obtained in the step (1) and not containing a bright material. A step (3) of applying a clear coating film on the colored base coating film obtained in the step (2) to form a clear coating film, and a step (1), (2) and (3) obtained in the above steps (1), (2) and (3). A method for forming a multi-layer coating film, which comprises a step (4) of heat-curing the metallic base coating film, the coloring base coating film, and the clear coating film to form a multi-layer coating film, which is obtained in the step (1). The light reflectance of the single metallic base coating film obtained by heating and curing the metallic base coating film as a single film is 45 to 50% at wavelengths of 650 to 700 nm and 20% or less at wavelengths of 410 to 440 nm and 510 to 590 nm. The light transmittance of the single-colored base coating film obtained by heating and curing the colored base coating film obtained in the step (2) as a single film is 50 to 70% at a wavelength of 400 to 700 nm and a wavelength of 650 to 650. A method for forming a high-design multilayer coating film, which is 88 to 92% at 700 nm and 20 to 60% at wavelengths of 410 to 440 nm and 510 to 590 nm, is disclosed.

Japanese Unexamined Patent Publication No. 2014-42891

However, according to the study by the present inventors, the method described in Patent Document 1 may not provide sufficient designability or may cause color variation due to film thickness variation.

The present invention has been made in view of the above-mentioned conventional circumstances, and a highly-designed coating film having high saturation, excellent brilliance and depth, and suppressed color variation due to film thickness variation can be obtained. It is an object to provide a coating method that can be used.

As a result of diligent studies to achieve the above object, the present inventors have found that the above problems can be solved by the following coating method, and have completed the present invention.
[1] Includes a step of applying an electrodeposition coating material (EP) to an outer plate portion of an automobile body to form an electrodeposition coating film (EC1), and then forming a first multilayer coating film on the outer plate portion. , How to paint the car body,
The step of forming the first multilayer coating film is
(1a) An intermediate coating film (PCo) forming step of coating a thermosetting intermediate coating film (PP) on the electrodeposition coating film (EC1) to form an intermediate coating film (PCo).
(1b) A first base coating film (BC1o) is formed by coating a thermosetting first base coating film (BP1) on the intermediate coating film (PCo) to form an uncured first base coating film (BC1o). BC1o) Formation process,
(1c) A second base coating film (BC2o) that is thermosetting is coated on the uncured first base coating film (BC1o) to form an uncured second base coating film (BC2o). Base coating (BC2o) forming process,
(1d) A clear coating film forming step of coating a thermosetting clear coating film on the uncured second base coating film (BC2o) to form an uncured clear coating film, and (1e) the uncured The first base coating film (BC1o), the uncured second base coating film (BC2o), and the uncured clear coating film are heated, and the baking step of simultaneously curing these coating films is included.
A method of painting an automobile body that satisfies the following conditions (A) to (F).
(A) ^{L *} value of the intermediate coating film (PCo) and ^(L * ^(PCo)), the absolute value of the difference between the first base coating ^{L *} value of the membrane ^{(BC1o) (L * (BC1o} )) | L ^* (PCo) -L ^* (BC1o) | is 35 or less.
(B) The first base paint (BP1) and the second base paint (BP2) do not contain a scaly aluminum pigment.
(C) the first base coating film (BC1o) of ^L * ^{C *} h Table hue angle of the color system chromaticity diagram and (h (BC1o)), the second base coating film (BC2o) ^L * ^C * h The absolute value | h (BC1o) -h (BC2o) | of the difference from the hue angle (h (BC2o)) in the color chromaticity diagram is 16 or less.
(D) The second base paint (BP2) contains a scaly mica pigment (P1) and a coloring pigment (P2), and the content of the scaly mica pigment (P1) is the second base paint (P1). It is 0.1% by mass or more and less than 3% by mass based on the solid content of BP2).
(E) The light transmittance of the second base coating film (BC2o) at a wavelength of 400 to 700 nm is 40% or more and less than 60%.
(F) The cured film thickness of the second base coating film (BC2o) is 4 to 12 μm.
[2] Further, a step of applying the electrodeposition coating material (EP) to the inner plate portion of the automobile body to form an electrodeposition coating film (EC2), and then forming a second coating film on the inner plate portion. Including
The step of forming the second coating film is
(2a) The first base coating film (BC1i) is formed by coating the first base coating film (BP1) on the electrodeposition coating film (EC2) to form an uncured first base coating film (BC1i). A step and (2b) a baking step of heating and curing the uncured first base coating film (BC1i) are included.
The method for painting an automobile body according to [1], which satisfies the following condition (G).
(G) Absolute value of the difference between the ^{L *} value (L ^* (EC2)) of the electrodeposition coating film (EC2) and the L ^* value (L ^{* (BC1i)) of the first base coating film (BC1i).} | L ^* (EC2) -L ^* (BC1i) | is 30 or less.
[3] The method for painting an automobile body according to [1] or [2], which further satisfies the following condition (H).
(H) The first base paint (BP1) has 5 to 25 parts by mass of a coloring pigment with respect to 100 parts by mass of the total resin solid content in the first base paint (BP1).

According to the coating method of the present invention, a highly-designed coating film having high saturation, excellent brilliance and depth, and suppressed color variation due to film thickness variation can be obtained.

Hereinafter, the present invention will be described in detail, but these show examples of desirable embodiments, and the present invention is not specified in these contents.

<Painting on the outer panel of the car body>
In the coating method of the present invention, an electrodeposition coating film (EP) is applied to an outer panel portion (hereinafter, may be simply referred to as "outer panel portion") of an automobile body to form an electrodeposition coating film (EC1). After that, the step of forming the first multilayer coating film on the outer plate portion is included.

The outer panel portion is a portion that can be visually recognized from the outside of the completed vehicle that has completed the mounting process.
The outer plate portion is usually formed of a steel plate such as a cold-rolled steel plate, a galvanized steel plate, a galvanized steel plate, a stainless steel plate, or a tin-plated steel plate.

Further, the steel sheet may be subjected to chemical conversion treatment before being coated with the electrodeposition coating material (EP). Examples of the chemical conversion treatment include phosphate treatment such as zinc phosphate treatment and iron phosphate treatment, composite oxide film treatment, chromium phosphate treatment, chromate treatment and the like.

[Step of forming an electrodeposition coating film (EC1) on the outer panel of an automobile body]
In the present invention, first, an electrodeposition coating material (EP) is applied to an outer panel portion of an automobile body to form an electrodeposition coating film (EC1).
The electrodeposition coating film (EC1) is formed mainly for the purpose of imparting corrosion resistance.

As the electrodeposition coating material (EP), it is preferable to use a cationic electrodeposition coating material.
The electrodeposition coating film (EC1) formed by the cationic electrodeposition coating material is, for example, diluted with deionized water or the like to have a solid content concentration of 5 to 40% by mass, and the pH is within the range of 5.5 to 9.0. The electrodeposition bath made of the cationic electrodeposition coating material adjusted to the above can be carried out by adjusting the bath temperature to 15 to 35 ° C. and energizing the outer plate portion as a cathode under the condition of a load voltage of 100 to 400 V.

As the cationic electrodeposition coating material, for example, a known coating material obtained by blending an aqueous solution or dispersion of a cationic polymer compound with a cross-linking agent, various pigments, and other additives, if necessary, can be used. ..

Examples of the cationic polymer compound include those in which an amino group is introduced into an acrylic resin or an epoxy resin having a crosslinkable functional group such as a hydroxyl group. By neutralizing this with an organic acid, an inorganic acid or the like to make it water-soluble or aqueous-dispersed, an aqueous solution or dispersion of a cationic polymer compound can be obtained.
As the cross-linking agent, a blocked polyisocyanate compound or an alicyclic epoxy compound can be used.

After electrodeposition coating, wash with ultrafiltration solution (UF filtrate), reverse osmosis permeation water (RO water), industrial water, pure water, etc. to remove excess cationic electrodeposition paint adhering to the outer panel. May be good.

The coating film is heat-cured by electrodeposition coating at a temperature of 110 to 200 ° C., preferably 140 to 180 ° C., using a drying facility such as an electric hot air dryer or a gas hot air dryer. The coating can be heated for 180 minutes, preferably 20 to 50 minutes.

The coating film thickness of the cationic electrodeposition coating material is, for example, 10 to 30 μm, preferably 15 to 25 μm as a dry film thickness.

[Step of forming the first multilayer coating film]
The step of forming the first multilayer coating film includes the steps (1a) to (1e) described below.

[Step (1a)]
In the step (1a) of the present invention, a thermosetting intermediate coating film (PP) is coated on the electrodeposition coating film (EC1) to form an intermediate coating film (PCo).
The intermediate coating film (PCo) is formed to conceal the electrodeposition coating film (EC1) and to impart adhesiveness, chipping resistance, etc. to the outer plate portion.

The intermediate coating paint (PP) contains a resin component, a curing agent, and the like. The intermediate coating material (PP) can be prepared, for example, by dissolving or dispersing a resin component, a curing agent, or the like in water or an organic solvent.

Examples of the resin component include a resin having a functional group (for example, a hydroxyl group, a carboxysil group, etc.) that reacts with a curing agent. Specific examples thereof include known resins such as acrylic resin, polyester resin, alkyd resin, chlorinated polyolefin resin, epoxy resin, and polyurethane resin.

Examples of the curing agent include melamine resin, polyisocyanate compound, blocked polyisocyanate compound, carbodiimide group-containing compound and the like.

If necessary, the intermediate coating paint (PP) may be further added with known components such as a curing catalyst, a coloring pigment, an extender pigment, and other paint additives in an appropriate ratio in addition to the above components. it can.

Examples of the method for applying the intermediate coating paint (PP) include an air spray method, an airless spray method, and an electrostatic coating method.
The film thickness of the intermediate coating film (PCo) is, for example, 10 to 50 μm, preferably 13 to 40 μm as the cured film thickness.

The intermediate coating film (PCo) may be heat-cured before applying the thermosetting first base paint (BP1) described later, or may be heat-cured in the uncured state of the thermosetting first base paint (BP1). May be painted.

When the intermediate coating film (PCo) is heat-cured, the heating temperature is preferably 80 to 160 ° C, more preferably 100 to 150 ° C. The heating time is preferably 10 to 60 minutes, more preferably 15 to 40 minutes.

Further, when the thermosetting first base paint (BP1) described later is applied in a state where the intermediate coating film (PCo) is uncured, it may be allowed to stand after the intermediate coating coating film (PP) is applied, or it may be known. Preheating can be performed by means. Preheating can be performed, for example, at 40 to 90 ° C., preferably 50 to 85 ° C., for example, for 1 to 10 minutes, preferably for 2 to 7 minutes.

From the viewpoint of the design of the obtained coating film, the intermediate coating film (PCo) has ^{an L *} value (L ^* (PCo)), which is the brightness in ^{the L *} a ^* b ^* color system, within the range of 40 to 65. It is preferably in the range of 45 to 60, and more preferably in the range of 45 to 60.

The above L ^* a ^* b ^* color system is a color system standardized by the International Commission on Illumination (CIE) in 1976 and specified in JIS Z 8781-4 (2013) in Japan as well. L ^* , chromaticity indicating hue and saturation are represented by a ^* and b ^* .

^{The L *} value in the present specification is an irradiation light of 45 degrees with respect to the vertical axis of the coating film surface using a spectrophotometer CM512m3 (trade name, manufactured by Konica Minolta Co., Ltd.) with respect to the coating film surface. It is defined as a numerical value calculated from the spectral reflectance received at 90 degrees.

Further, when the thermosetting first base paint (BP1) described later is applied in a state where the intermediate coating film (PCo) is uncured, the L ^* value (L ^* (PCo)) of the intermediate coating film (PCo) is applied. ^{) Shall use the L *} value (L ^* (PCo)) measured for the intermediate coating film (PCo) obtained by separately coating the intermediate coating paint (PP) under the same conditions and curing by heating.

^{The L *} value (L ^* (PCo)) of the intermediate coating film (PCo) is, for example, a pigment such as a black pigment, a white pigment or other coloring pigment, or an extender pigment, which is blended in the intermediate coating coating (PP). It can be appropriately adjusted by adjusting the blending amount and blending ratio.

[Step (1b)]
In the step (1b) of the present invention, a thermosetting first base coating film (BP1) is coated on the intermediate coating film (PCo) to form an uncured first base coating film (BC1o). ..
The first base paint (BP1) usually contains a coloring pigment.

As the coloring pigment, one kind or a combination of two or more kinds of coloring pigments known for coatings can be used. For example, metal oxide pigments such as titanium oxide and iron oxide, and composite metal oxide pigments such as titanium yellow. , Carbon black, azo pigments, quinacridone pigments, diketopyrrolopyrrole pigments, perylene pigments, perinone pigments, benzimidazolone pigments, isoindolin pigments, isoindolinone pigments, metal chelate azo pigments, phthalocyanine Examples thereof include based pigments, indanslon pigments, dioxane pigments, and indigo pigments.

The content of the coloring pigment in the first base coating material (BP1) is based on 100 parts by mass of the total resin solid content in the first base coating material (BP1) from the viewpoint of the coloring power of the obtained coating film and the appearance of the finished product. It is preferably 5 to 25 parts by mass, more preferably 10 to 20 parts by mass, and even more preferably 13 to 18 parts by mass.

The first base paint (BP1) can contain the above-mentioned resin component, the above-mentioned curing agent, and the like in addition to the coloring pigment.

In addition to the above components, for example, an organic solvent, a curing catalyst, an extender pigment, a rheology control agent, a pigment dispersant, a settling inhibitor, an antifoaming agent, and an antioxidant are added to the first base coating material (BP1), if necessary. Known components such as agents, UV absorbers, and other paint additives can be added in appropriate proportions.

Further, the first base paint (BP1) does not contain a scaly aluminum pigment. By not containing the scaly aluminum pigment, it is possible to form a coating film having high saturation and suppressing color fluctuation due to film thickness fluctuation.

The first base coating material (BP1) can be prepared by mixing and dispersing the above-mentioned components.

Examples of the method for applying the first base paint (BP1) include an air spray method, an airless spray method, an electrostatic coating method and the like.
The film thickness of the first base coating film (BC1o) is, for example, 5 to 15 μm, preferably 6 to 13 μm as the cured film thickness.

^{Further, in the present invention, the difference between the L *} value (L ^* (PCo)) of the intermediate coating film (PCo) and the L ^* value (L ^* (BC1o)) of the first base coating film (BC1o). Absolute value | L ^* (PCo) -L ^* (BC1o) | is 35 or less.
When | L ^* (PCo) -L ^* (BC1o) | is 35 or less, color variation due to film thickness variation of the multi-layer coating film can be suppressed.

| L ^* (PCo) -L ^* (BC1o) | is preferably 34 or less, more preferably 33 or less, from the viewpoint of suppressing color fluctuation of the multilayer coating film.

^{In the present invention, the L *} value (L ^* (BC1o)) of the first base coating film (BC1o) is a coating film on which ^{a gray (L *} value = 64) cured coating film is formed. The first base paint (BP1) is applied so that the film thickness is 10 μm, and the cured coating film is obtained by heating at 140 ° C. for 30 minutes. JIS Z8781-4 (2013) calculated from the spectral reflectance received at 90 degrees to the coating film surface with irradiation light of 45 degrees to the vertical axis of the coating film surface using Minolta Co., Ltd. It means ^{the L *} value as the brightness ^{in the L *} a ^* b ^* color system specified in (year).

In order to set | L ^* (PCo) -L ^* (BC1o) | in the above range, the components and contents of the intermediate coating material (PP) and the first base coating material (BP1) may be appropriately adjusted. More specifically, the type and content of the pigment in the paint may be appropriately adjusted.

[Step (1c)]
In the step (1c) of the present invention, a thermosetting second base coating film (BP2) is coated on the uncured first base coating film (BC1o), and the uncured second base coating film (BC2o) is applied. ) Is formed.
The second base paint (BP2) contains a scaly mica pigment (P1). The scaly mica pigment (P1) can improve the brilliance of the multi-layer coating film and enhance the saturation of the multi-layer coating film.

Examples of the scaly mica pigment (P1) include scaly natural mica and scaly artificial mica.
Natural mica is crushed ore mica. The artificial _{mica, SiO 2, MgO, Al 2} 0 3, K 2 SiF 6, by heating the industrial materials such as Na 2 SiF _6, and melted at a high temperature of about 1500 ° C., was synthesized cooled to crystallize It is a thing.

Specific examples of the artificial mica include fluorine gold mica (KMg ₃ AlSi ₃ O ₁₀ F ₂ ), potassium tetrasilicon mica (KMg ₂₅ AlSi ₄ O ₁₀ F ₂ ), and sodium tetrasilicon mica (NaMg ₂₅ AlSi ₄ O _10). F ₂ ), Na teniolite (NaMg ₂ LiSi ₄ O ₁₀ F ₂ ), LiNa teniolite (LiMg ₂ LiSi ₄ O ₁₀ F ₂ ) and the like are known.

The artificial mica may be surface-treated. Examples of the surface-treated artificial mica include an interference mica whose surface is coated with titanium dioxide or the like, a colored mica whose surface is chemically adsorbed with a coloring pigment, or whose surface is coated with iron oxide.

The content of the scaly mica pigment (P1) in the second base paint (BP2) is 0.1% by mass or more and less than 3% by mass based on the solid content of the second base paint (BP2). When the content is 0.1% by mass or more, sufficient brilliance can be given to the obtained coating film. When the content is less than 3% by mass, the color of the first base coating film (BC1o) is hidden due to the influence of the scaly mica pigment (P1) even if the film thickness of the second base coating film (BC2o) becomes thick. Since it can be suppressed, it is possible to suppress color fluctuation due to film thickness variation of the multi-layer coating film.

From the viewpoint of the brilliance of the obtained coating film, the content is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, based on the solid content of the second base coating material (BP2). Is. Further, the content is preferably 2.8% by mass or less, more preferably 2.7% by mass or less, from the viewpoint of suppressing color fluctuation due to the film thickness variation of the multi-layer coating film.

The second base paint (BP2) contains a coloring pigment (P2). The coloring pigment (P2) can improve the color depth of the first base coating film (BC1o).
As the coloring pigment (P2), the same one as that used for the first base coating material (BP1) can be used.

The content of the coloring pigment (P2) in the second base coating material (BP2) is preferably 0 based on the solid content of the second base coating material (BP2) from the viewpoint of the coloring power of the obtained coating film and the appearance of the finished product. It is 1 to 3.0% by mass, more preferably 0.5 to 2.5% by mass, and further preferably 1.0 to 2.0% by mass.

The second base paint (BP2) can contain the above-mentioned resin component, the above-mentioned curing agent, and the like, in addition to the scaly mica pigment (P1) and the coloring pigment (P2).

The content of the curing agent in the second base coating material (BP2) is preferably 100 parts by mass with respect to 100 parts by mass of the total resin solid content in the second base coating material (BP2) from the viewpoint of coating durability and shortening of curing time. Is 10 to 70 parts by mass, more preferably 15 to 50 parts by mass, and even more preferably 20 to 40 parts by mass.

In addition to the above components, for example, an organic solvent, a curing catalyst, an extender pigment, a rheology control agent, a pigment dispersant, a settling inhibitor, an antifoaming agent, and an antioxidant are added to the second base coating material (BP2), if necessary. Known components such as agents, UV absorbers, and other paint additives can be added in appropriate proportions.

Further, the second base paint (BP2) does not contain a scaly aluminum pigment. By not containing the scaly aluminum pigment, it is possible to form a coating film having excellent saturation and a sense of depth and suppressing color fluctuation due to film thickness fluctuation.

The second base coating material (BP2) can be prepared by mixing and dispersing the above-mentioned components.

Examples of the method for applying the second base paint (BP2) include an air spray method, an airless spray method, an electrostatic coating method and the like.

The film thickness of the second base coating film (BC2o) is 4 to 12 μm as a cured film thickness. When the film thickness is 4 μm or more, the color depth of the first base coating film (BC1o) can be sufficiently improved. When the film thickness is 12 μm or less, the width of the film thickness of the multi-layer coating film does not become too large, and color variation due to film thickness variation can be suppressed.

The film thickness of the second base coating film (BC2o) is preferably 5 μm or more as the cured film thickness from the viewpoint of improving the color depth of the first base coating film (BC1o). The film thickness is preferably 7 μm or less from the viewpoint of suppressing fluctuations in the film thickness of the multi-layer coating film.

In the present invention, the hue angle of the ^L * ^{C *} h color system chromaticity diagram of the first base coating film (BC1o) (h (BC1o) ), the second base coating film (BC2o) ^L * C ^* The absolute value | h (BC1o) -h (BC2o) | of the difference from the hue angle (h (BC2o)) in the h color chromaticity diagram is 16 or less.

When | h (BC1o) -h (BC2o) | is 16 or less, the difference in hue between the first base coating film (BC1o) and the second base coating film (BC2o) is small, so that the film thickness of the multi-layer coating film is small. Color fluctuation due to fluctuation can be suppressed.

| H (BC1o) -h (BC2o) | is preferably 14 or less, more preferably 12 or less, from the viewpoint of suppressing color fluctuation of the multilayer coating film.

The hue angle of the L ^* C ^* h chromaticity diagram can be measured by, for example, a multi-angle spectrophotometer.
The L ^* C ^* h color system was devised based on the ^{L *} a ^* b ^* color system, which was defined by the International Commission on Illumination in 1976 and adopted in JIS Z8781-4 (2013). It is a color system. The hue angle in the L ^* C ^* h chromaticity diagram is an angle moved counterclockwise with the red axis as 0 ° in the chromaticity diagram.

^{In the present invention, a cured coating film of gray (L *} value = 64) was formed for the hue angle (h (BC1o)) of ^{the L *} C ^* h color system chromaticity diagram of the first base coating film (BC1o). The first base paint (BP1) is coated on the coated plate so that the thickness of the cured coating film is 10 μm, and the cured coating film is heated at 140 ° C. for 30 minutes to obtain a cured coating film. Calculated from the spectral reflectance received at 90 degrees with respect to the coating film surface with irradiation light of 45 degrees with respect to the vertical axis of the coating film surface using a photometric meter CM512m3 (trade name, manufactured by Konica Minolta Co., Ltd.). L ^* C ^* h means the _{hue angle h ab} in the color system.

^{Further, in the present invention, a cured coating film of gray (L *} value = 64) is formed as the hue angle (h (BC2o)) of ^{the L *} C ^* h color system chromaticity diagram of the second base coating film (BC2o). A second base paint (BP2) is applied onto the coated plate so that the thickness of the cured coating film is 6 μm, and the cured coating film is heated at 140 ° C. for 30 minutes to obtain a cured coating film. , Spectral reflectance received at 90 degrees with respect to the coating film surface with irradiation light of 45 degrees with respect to the vertical axis of the coating film surface using a spectrophotometer CM512m3 (trade name, manufactured by Konica Minolta Co., Ltd.). It means the hue angle h _ab in the L ^* C ^{* h color system calculated from.}

In order to make | h (BC1o) -h (BC2o) | within the above range, the components and contents of the first base paint (BP1) and the second base paint (BP2) may be appropriately adjusted. More specifically, the type and content of the pigment in the paint may be appropriately adjusted.

Further, in the present invention, the light transmittance of the second base coating film (BC2o) at a wavelength of 400 to 700 nm is 40% or more and less than 60%. When the light transmittance is 40% or more, a multi-layer coating film having excellent depth and brilliance can be obtained. When the light transmittance is less than 60%, it is possible to obtain a multi-layer coating film having an excellent sense of depth and suppressing color fluctuation due to film thickness fluctuation.

The light transmittance is preferably 43% or more, more preferably 45% or more, from the viewpoint of improving the color depth of the first base coating film (BC1o). The light transmittance is preferably 57% or less, more preferably 53% or less, from the viewpoint of suppressing deterioration of the multi-layer coating film by light.

The light transmittance can be measured as follows.
That is, the second base coating material (BP2) is coated on a polypropylene plate so that the film thickness of the cured coating film is 6 μm, and heated at 140 ° C. for 30 minutes to obtain a cured coating film. Next, the cured coating film is peeled off from the polypropylene plate, and the average light transmittance in the wavelength range of 400 to 700 nm is measured for the cured coating film using a spectrophotometer.

In order to keep the light transmittance within the above range, the components and contents of the second base coating material (BP2) may be appropriately adjusted. More specifically, the type and content of the pigment in the paint may be appropriately adjusted.

[Step (1d)]
In the step (1d) of the present invention, a thermosetting clear coating film is applied onto the uncured second base coating film (BC2o) to form an uncured clear coating film.
The clear paint can improve the smoothness and vividness of the multi-layer coating film.

As the clear paint, a solvent-based and two-component thermosetting clear paint composition known for painting an automobile body can be used.
The solvent system is a term that is contrasted with water-based and means that the solvent does not substantially contain water.
Examples of the two-component thermosetting clear coating composition include an organic solvent-type thermosetting coating composition containing a two-component resin (main agent) having a crosslinkable functional group and a crosslinking agent. It can be used by mixing the above two liquids immediately before painting.

Examples of the crosslinkable functional group contained in the substrate resin include a carboxyl group, a hydroxyl group, an epoxy group and a silanol group.

Examples of the type of the base resin include acrylic resin, polyester resin, alkyd resin, urethane resin, epoxy resin, and fluororesin.

Examples of the cross-linking agent include polyisocyanate compounds, blocked polyisocyanate compounds, melamine resins, urea resins, carboxyl group-containing compounds, carboxyl group-containing resins, epoxy group-containing resins and epoxy group-containing compounds.

The ratio of the base resin and the cross-linking agent used is not particularly limited, but in general, the cross-linking agent is 10 to 100% by mass, preferably 20 to 80% by mass, and more preferably 30 to 30% by mass with respect to the total amount of the solid content of the base resin. It can be used within the range of 60% by mass.

If necessary, the clear paint can contain a coloring pigment, a brilliant pigment, a dye, etc. to the extent that the transparency is not impaired, and further, an extender pigment, an ultraviolet absorber, a light stabilizer, and a defoaming agent. , Thickener, rust preventive, surface conditioner and the like can be appropriately contained.

Examples of the method for applying the clear paint include an air spray method, an airless spray method, and an electrostatic coating method.

The film thickness of the clear coating film is, for example, 10 to 60 μm, preferably 25 to 50 μm as the cured film thickness.

[Step (1e)]
In the step (1e) of the present invention, the uncured first base coating film (BC1o), the uncured second base coating film (BC2o), and the uncured clear coating film are heated to obtain these coating films. Are cured at the same time.

The heating temperature is preferably 80 to 170 ° C., more preferably 120 to 160 ° C. from the viewpoint of obtaining a multi-layer coating film having excellent hardness.
The heating time is preferably 10 to 60 minutes, more preferably 20 to 40 minutes, from the viewpoint of obtaining a multi-layer coating film having excellent hardness.

As the heating method, ordinary coating film baking means, for example, hot air heating, infrared heating, high frequency heating, or the like can be adopted.

The film thickness of the first multilayer coating film is, for example, 60 to 110 μm, preferably 70 to 100 μm as the cured film thickness.

<Painting on the inner plate of the car body>
In the coating method of the present invention, an electrodeposition coating film (EP) is further applied to an inner plate portion (hereinafter, may be simply referred to as “inner plate portion”) of an automobile body to form an electrodeposition coating film (EC2). It is preferable to include a step of forming a second coating film, which will be described later, on the inner plate portion after forming the above.

When the coating method of the present invention includes the step of forming the second coating film described later, the color variation due to the film thickness variation is suppressed even when the base paint used for coating the outer plate portion is used in the inner plate portion. A coating film is obtained.

The inner plate portion is a portion that cannot be seen from the outside of the completed vehicle that has completed the mounting process.
The inner plate portion is formed of a steel plate similar to the outer plate portion.

[Step of forming an electrodeposition coating film (EC2) on the inner plate of an automobile body]
In the formation of the second coating film, first, the electrodeposition coating film (EP) is applied to the inner plate portion of the automobile body to form the electrodeposition coating film (EC2).
The electrodeposition coating film (EC2) is formed mainly for the purpose of imparting corrosion resistance.

As the electrodeposition coating material (EP), it is preferable to use a cationic electrodeposition coating material.
The electrodeposition coating film (EC2) formed by the cationic electrodeposition coating material is, for example, diluted with deionized water or the like to have a solid content concentration of 5 to 40% by mass, and the pH is within the range of 5.5 to 9.0. The electrodeposition bath made of the cationic electrodeposition coating material adjusted to the above can be carried out by adjusting the bath temperature to 15 to 35 ° C. and energizing the inner plate portion as a cathode under the condition of a load voltage of 100 to 400 V.

As the cationic electrodeposition coating material, for example, a known coating material obtained by blending an aqueous solution or dispersion of a cationic polymer compound with a cross-linking agent, various pigments, and other additives, if necessary, can be used. ..

Examples of the cationic polymer compound include those in which an amino group is introduced into an acrylic resin or an epoxy resin having a crosslinkable functional group such as a hydroxyl group. By neutralizing this with an organic acid, an inorganic acid or the like to make it water-soluble or aqueous-dispersed, an aqueous solution or dispersion of a cationic polymer compound can be obtained.
As the cross-linking agent, a blocked polyisocyanate compound or an alicyclic epoxy compound can be used.

After electrodeposition coating, wash with ultrafiltration solution (UF filtrate), reverse osmosis permeation water (RO water), industrial water, pure water, etc. to remove excess cationic electrodeposition paint adhering to the inner plate. May be good.

The coating film is heat-cured by electrodeposition coating at a temperature of 110 to 200 ° C., preferably 140 to 180 ° C., using a drying facility such as an electric hot air dryer or a gas hot air dryer. The coating can be heated for 180 minutes, preferably 20 to 50 minutes.

The coating film thickness of the cationic electrodeposition coating material is, for example, 10 to 30 μm, preferably 15 to 25 μm as a dry film thickness.

From the viewpoint of reducing the number of coating steps, it is preferable that the step of forming the electrodeposition coating film (EC2) is performed at the same time as the step of forming the electrodeposition coating film (EC1).

[Step of forming the second coating film]
The step of forming the second coating film includes the steps (2a) to (2b) described below.

[Step (2a)]
In the step (2a) of the present invention, a thermosetting first base coating film (BP1) is coated on the electrodeposition coating film (EC2) to form an uncured first base coating film (BC1i). ..

In the present invention, when forming the second coating film on the inner plate portion, the coating of the intermediate coating film (PP), the second base coating film (BP2) and the clear coating film is omitted, and after the electrodeposition coating film (EC2) is formed. A method of painting the first base paint (BP1) is adopted.
By adopting this method, the cost of the inner plate coating process can be reduced.

Further, as the first base paint (BP1), the above-mentioned one can be used.
By using the same first base paint as that used for the outer plate portion for the inner plate portion, it is possible to reduce the types of paint used and reduce the cost of the inner plate portion coating process.

Examples of the method for applying the first base paint (BP1) include an air spray method, an airless spray method, an electrostatic coating method and the like.
The film thickness of the first base coating film (BC1i) is, for example, 5 to 20 μm, preferably 8 to 15 μm as the cured film thickness.

Further, in the present invention, the difference between the L ^* value (L ^* (EC2)) of the electrodeposition coating film (EC2) and the L ^* value (L ^* (BC1i)) of the first base coating film (BC1i). Absolute value | L ^* (EC2) -L ^* (BC1i) | is 30 or less.

When | L ^* (EC2) -L ^* (BC1i) | is 30 or less, the difference in brightness between the electrodeposition coating film (EC2) and the first base coating film (BC1i) is small, so that the film thickness of the multi-layer coating film is small. Color variation due to thickness variation can be suppressed.

| L ^* (EC2) -L ^* (BC1i) | is preferably 28 or less, more preferably 25 or less, from the viewpoint of suppressing color fluctuation of the multi-layer coating film.

^{In the present invention, the L *} value (L ^* (BC1i)) of the first base coating film (BC1i) is a coating film on which ^{a gray (L *} value = 64) cured coating film is formed. The first base paint (BP1) is applied so that the film thickness is 10 μm, and the cured coating film is obtained by heating at 140 ° C. for 30 minutes. JIS Z8781-4 (2013) calculated from the spectral reflectance received at 90 degrees to the coating film surface with irradiation light of 45 degrees to the vertical axis of the coating film surface using Minolta Co., Ltd. It means ^{the L *} value as the brightness ^{in the L *} a ^* b ^* color system specified in (year).

In order to make | L ^* (EC2) -L ^* (BC1i) | within the above range, the components and contents of the electrodeposition coating film (EC2) and the first base coating film (BP1) may be appropriately adjusted. .. More specifically, the type and content of the pigment in the paint may be appropriately adjusted.

[Step (2b)]
In the step (2b) of the present invention, the uncured first base coating film (BC1i) is heated and cured. The baking step in the step (2b) is preferably performed at the same time as the baking step in the step (1e) from the viewpoint of energy saving and the like.

The heating temperature of the uncured first base coating film (BC1i) is preferably 80 to 170 ° C., more preferably 120 to 160 ° C. from the viewpoint of obtaining a multi-layer coating film having excellent hardness.
The heating time is preferably 10 to 60 minutes, more preferably 20 to 40 minutes, from the viewpoint of obtaining a multi-layer coating film having excellent hardness.

As the heating method, ordinary coating film baking means, for example, hot air heating, infrared heating, high frequency heating, or the like can be adopted.

Hereinafter, the present invention will be described in more detail with reference to examples. The scope of the present invention is not limited to these examples. Moreover, "part" in an Example indicates "mass part".

[Test board]
A zinc phosphate-treated steel sheet (450 mm × 300 mm × 0.8 mm) was coated with a cationic electrodeposition paint EP (trade name “Electron 9900”, manufactured by Kansai Paint Co., Ltd.). The steel sheet after electrodeposition coating was baked and dried at 170 ° C. for 30 minutes to prepare a steel sheet having an electrodeposition coating film (EC) having a thickness of 20 μm.
Thus, a test plate was obtained assuming the outer plate portion and the inner plate portion after electrodeposition coating.

^{The L *} a ^* b ^{* L *} value (L ^* (EC)) as the brightness in the color system of this electrodeposition coating film (EC) specified in JIS Z8781-4 (2013) is measured by a spectrophotometer (L * (EC)). Calculated from the spectral reflectance received at 90 degrees to the coating film surface with irradiation light of 45 degrees with respect to the vertical axis of the coating film surface using the product name "CM-512m3", manufactured by Konica Minolta). As a result, it was 52.

[Intermediate paint]
As a resin component, an organic solvent type intermediate coating PP (trade name "HS30 D60", manufactured by Kansai Paint Co., Ltd.) containing an acrylic resin, a polyester resin and a melamine resin was prepared.

[1st base paint]
(1st base paint (BP1-1))
As a resin component, an organic solvent type base paint (trade name "DN60", manufactured by Kansai Paint Co., Ltd.) containing an acrylic resin, a polyester resin and a melamine resin was prepared.
Using the above base paint, 11 parts of "RED L3661HD" (diketopyrrolopyrrole red pigment, trade name, manufactured by BASF), "Todacolor KN-O" for 100 parts of the total resin solid content in the base paint. "(Iron oxide pigment, trade name, manufactured by Toda Kogyo Co., Ltd.) 4 parts," TRONOX CR-826 "(titanium oxide pigment, trade name, manufactured by TRONOX Co., Ltd.) 0.6 parts and" Carbon MA-100 "(carbon black pigment) , Trade name, manufactured by Mitsubishi Chemical Co., Ltd.) An organic solvent type first base paint (BP1-1) containing 0.01 part was prepared.

(1st base paint (BP1-2)) to (1st base paint (BP1-8))
An organic solvent type base paint (trade name "DN60", manufactured by Kansai Paint Co., Ltd.) was prepared.
Using the base paint, the content of each pigment is the amount shown in Tables 1 to 4 with respect to 100 parts of the total resin solid content in the base paint (BP1-2). )-(BP1-8) were prepared.
As the aluminum pigment, "GX180A" (trade name, manufactured by Asahi Kasei Metals Co., Ltd.) was used.

[Second base paint]
(2nd base paint (BP2-1))
An organic solvent type base paint (trade name "DN60", manufactured by Kansai Paint Co., Ltd.) was prepared.
Using the above base paint, 0.7 parts of "ME CFS FINE RED 4303V" (scaly mica pigment, trade name, manufactured by BASF) with respect to 100 parts of the total resin solid content in the base paint, 1.6 copies of "ME SUPER GOLD 239Z" (scaly mica pigment, trade name, manufactured by BASF) and 1.5 copies of "R6436" (perylene red pigment, trade name, manufactured by Sun Chemical Corporation) An organic solvent type second base coating material (BP2-1) containing the above was prepared.

(Second base paint (BP2-2) to (BP2-9))
An organic solvent type base paint (trade name "DN60", manufactured by Kansai Paint Co., Ltd.) was prepared.
Using the base paint, the content of each pigment is the amount shown in Tables 1 to 4 with respect to 100 parts of the total resin solid content in the base paint (BP2-2). )-(BP2-9) were prepared.
As the aluminum pigment, "GX180A" (trade name, manufactured by Asahi Kasei Metals Co., Ltd.) was used.

[Example 1]
(Outer panel painting process)
The intermediate coating PP is applied to the test plate obtained above using a rotary atomization type electrostatic coating machine so that the curing film thickness is 35 μm, and the test plate is heated at 140 ° C. for 30 minutes to be cured. , An intermediate coating film (PCo) was obtained.
^{The L *} value (L ^* (PCo)) of this intermediate coating film (PCo) was measured on the vertical axis of the coating film surface using a spectrophotometer (trade name "CM-512m3", manufactured by Konica Minolta Co., Ltd.). On the other hand, it was 64 when calculated from the spectral reflectance received at 90 degrees with respect to the coating film surface with the irradiation light of 45 degrees.

The first base paint BP1-1 was applied onto an intermediate coating film (PCo) using a rotary atomization type electrostatic coating machine so that the cured film thickness was 10 μm, and allowed to stand for 90 seconds. 1 base coating film (BC1o) was obtained.

^{L *} a ^* b ^{* L *} value (L ^* (BC1o)) and L ^* C ^* h as lightness in the color system specified in JIS Z8781-4 (2013) of the first base coating film (BC1o). The hue angle (h (BC1o)) in the color chromaticity diagram was separately measured as follows. That is, the first base paint BP1-1 is coated on the coated plate on which the gray (L ^* value = 64) cured coating film is formed so that the thickness of the cured coating film is 10 μm, and the temperature is 140 ° C. for 30 minutes. After heating to obtain a cured coating film, the cured coating film is irradiated with 45 degree light with respect to the vertical axis of the coating film surface using a spectrophotometer CM512m3 (trade name, manufactured by Konica Minolta Co., Ltd.). Then, it was calculated from the spectral reflectance received at 90 degrees with respect to the coating film surface. The results are shown in Table 1.

Then, the second base coating film BP2-1 was applied onto the first base coating film (BC1o) using a rotary atomization type electrostatic coating machine so that the cured film thickness was 6 μm, and allowed to stand for 8 minutes. Then, a second base coating film (BC2o) was obtained.

The hue angle (h (BC2o)) of ^{the L *} C ^* h color chromaticity diagram of the second base coating film (BC2o) was separately measured as follows. That is, the second base paint BP2-1 is coated on the coated plate on which the gray (L ^* value = 64) cured coating film is formed so that the thickness of the cured coating film is 6 μm, and the temperature is 140 ° C. for 30 minutes. After heating to obtain a cured coating film, the cured coating film is irradiated with 45 degree light with respect to the vertical axis of the coating film surface using a spectrophotometer CM512m3 (trade name, manufactured by Konica Minolta Co., Ltd.). Then, it was calculated from the spectral reflectance received at 90 degrees with respect to the coating film surface. The results are shown in Table 1.

Further, the light transmittance of the second base coating film (BC2o) at a wavelength of 400 to 700 nm was measured as follows.
The second base coating material BP2-1 was applied to a polypropylene plate and cured to a film thickness of 6 μm, and the obtained coating film was peeled off. Using the coating film as a sample, the light transmittance was measured in the wavelength range of 400 to 700 nm using a spectrophotometer (trade name “UV3700”, manufactured by Shimadzu Corporation). The results are shown in Table 1.

After that, the acid epoxy clear paint (trade name "DKC-K12-1", manufactured by Kansai Paint Co., Ltd.) was applied to the second base using a rotary atomization type electrostatic coating machine so that the cured film thickness was 35 μm. It was applied on a coating film (BC2o) and allowed to stand for 10 minutes to obtain a clear coating film.

Then, the test plate after painting was heated at 140 ° C. for 30 minutes.

(Inner plate painting process)
The first base coating film BP1-1 was applied to the test plate obtained above using an air spray so that the cured film thickness was 10 μm, and allowed to stand for 18 minutes to obtain the first base coating film (BC1i). Got

^{The L *} a ^* b ^{* L *} value (L ^* (BC1i)) as the brightness in the color system specified in JIS Z8781-4 (2013) of the first base coating film (BC1i) and the above electrodeposition coating. The difference from the L ^* value (L ^* (EC)) of the film was calculated. The results are shown in Table 1.

^{The L *} a ^* b ^{* L *} value (L ^* (BC1i)) as the brightness in the color system of the first base coating film (BC1i) specified in JIS Z8781-4 (2013) is as follows. It was measured separately in this way. That is, the first base paint BP1-1 is coated on the coated plate on which the gray (L ^* value = 64) cured coating film is formed so that the thickness of the cured coating film is 10 μm, and the temperature is 140 ° C. for 30 minutes. After heating to obtain a cured coating film, the cured coating film is irradiated with 45 degree light with respect to the vertical axis of the coating film surface using a spectrophotometer CM512m3 (trade name, manufactured by Konica Minolta Co., Ltd.). Then, it was calculated from the spectral reflectance received at 90 degrees with respect to the coating film surface.

Then, the test plate after painting was heated at 140 ° C. for 30 minutes.

[Examples 2 to 10, Comparative Examples 1 to 9]
The first base paint BP1-1 and the second base paint BP2-1 are used as the first base paint or the second base paint shown in Tables 1 to 4, and the second base paint has a cured film thickness shown in Tables 1 to 4. In the same manner as in Example 1, the outer plate portion was coated and the inner plate portion was coated, and the same measurement was performed. The results are shown in Tables 1-4.

[Coating film evaluation]
(saturation)
The saturation of the multi-layer coating film on the outer plate portion obtained in the above Examples and Comparative Examples was evaluated by irradiating the test plate with an artificial sun lamp, visually observing it, and evaluating it according to the following criteria. The results are shown in Tables 1-4.

⊚: Shows excellent saturation.
◯: There is some saturation.
Δ: Almost no saturation.
×: There is no saturation at all.

(Brightness)
The brilliance of the multi-layer coating film on the outer plate portion obtained in the above Examples and Comparative Examples was evaluated by irradiating the test plate with an artificial sun lamp, visually observing it, and evaluating it according to the following criteria. The results are shown in Tables 1-4.

⊚: Shows excellent brilliance.
◯: Slightly brilliant.
Δ: Almost no brilliance.
X: There is no brilliance at all.

(A feeling of depth)
The color depth of the multi-layer coating film on the outer plate portion obtained in the above Examples and Comparative Examples was visually observed by irradiating the test plate with an artificial sun lamp. When a strong sense of depth and three-dimensionality was felt, the sense of depth was considered to be excellent, and evaluation was made according to the following criteria. The results are shown in Tables 1-4.

⊚: Shows an excellent sense of depth.
◯: There is a sense of depth.
Δ: There is almost no sense of depth.
×: There is no sense of depth at all.

(Color variation)
The color variation of the multi-layer coating film on the outer plate portion and the coating film on the inner plate portion obtained in the above Examples and Comparative Examples was evaluated according to the following criteria by visually observing the test plate. The results are shown in Tables 1-4.

⊚: Almost no color unevenness is observed, and the coating film has an extremely excellent appearance.
◯: Although slight color unevenness is observed, it has an excellent coating film appearance.
Δ: Color unevenness is observed, and the appearance of the coating film is slightly inferior.
X: Many color unevenness is observed, and the appearance of the coating film is inferior.

The cured film thickness of the coating film has an error of about ± 15% (film thickness variation) depending on the measurement location, and it is considered that the color unevenness is caused by the error.

From the results shown in Tables 1 to 4, the coating method of the present invention can obtain a highly-designed coating film having high saturation, excellent brilliance and depth, and suppressed color variation due to film thickness variation. Do you get it.

Claims (3)

An automobile body including a step of applying an electrodeposition paint (EP) to an outer plate portion of an automobile body to form an electrodeposition coating film (EC1) and then forming a first multilayer coating film on the outer plate portion. It is a painting method of
The step of forming the first multilayer coating film is
(1a) An intermediate coating film (PCo) forming step of coating a thermosetting intermediate coating film (PP) on the electrodeposition coating film (EC1) to form an intermediate coating film (PCo).
(1b) A first base coating film (BC1o) is formed by coating a thermosetting first base coating film (BP1) on the intermediate coating film (PCo) to form an uncured first base coating film (BC1o). BC1o) Formation process,
(1c) A second base coating film (BC2o) that is thermosetting is coated on the uncured first base coating film (BC1o) to form an uncured second base coating film (BC2o). Base coating (BC2o) forming process,
(1d) A clear coating film forming step of coating a thermosetting clear coating film on the uncured second base coating film (BC2o) to form an uncured clear coating film, and (1e) the uncured The first base coating film (BC1o), the uncured second base coating film (BC2o), and the uncured clear coating film are heated, and the baking step of simultaneously curing these coating films is included.
A method of painting an automobile body that satisfies the following conditions (A) to (F).
(A) ^{L *} value of the intermediate coating film (PCo) and ^(L * ^(PCo)), the absolute value of the difference between the first base coating ^{L *} value of the membrane ^{(BC1o) (L * (BC1o} )) | L ^* (PCo) -L ^* (BC1o) | is 35 or less.
(B) The first base paint (BP1) and the second base paint (BP2) do not contain a scaly aluminum pigment.
(C) the first base coating film (BC1o) of ^L * ^{C *} h Table hue angle of the color system chromaticity diagram and (h (BC1o)), the second base coating film (BC2o) ^L * ^C * h The absolute value | h (BC1o) -h (BC2o) | of the difference from the hue angle (h (BC2o)) in the color chromaticity diagram is 16 or less.
(D) The second base paint (BP2) contains a scaly mica pigment (P1) and a coloring pigment (P2), and the content of the scaly mica pigment (P1) is the second base paint (P1). It is 0.1% by mass or more and less than 3% by mass based on the solid content of BP2).
(E) The light transmittance of the second base coating film (BC2o) at a wavelength of 400 to 700 nm is 40% or more and less than 60%.
(F) The cured film thickness of the second base coating film (BC2o) is 4 to 12 μm. Further, the step of coating the electrodeposition coating material (EP) on the inner plate portion of the automobile body to form the electrodeposition coating film (EC2) and then forming the second coating film on the inner plate portion is included.
The step of forming the second coating film is
(2a) The first base coating film (BC1i) is formed by coating the first base coating film (BP1) on the electrodeposition coating film (EC2) to form an uncured first base coating film (BC1i). A step and (2b) a baking step of heating and curing the uncured first base coating film (BC1i) are included.
The method for painting an automobile body according to claim 1, which satisfies the following condition (G).
(G) Absolute value of the difference between the ^{L *} value (L ^* (EC2)) of the electrodeposition coating film (EC2) and the L ^* value (L ^{* (BC1i)) of the first base coating film (BC1i).} | L ^* (EC2) -L ^* (BC1i) | is 30 or less. The method for painting an automobile body according to claim 1 or 2, further satisfying the following condition (H).
(H) The first base paint (BP1) has 5 to 25 parts by mass of a coloring pigment with respect to 100 parts by mass of the total resin solid content in the first base paint (BP1).