KR100435941B1 - Method of forming multiple-layered coating film - Google Patents

Abstract

The present invention provides a method of forming a multilayer coating film by sequentially coating a primary coating (A), an intermediate coating (B) and a final coating (C) on a substrate,

(1) The intermediate coating paint (B), which contains 0.1 to 30 parts by weight of aluminum powder and 1 to 200 parts by weight of titanium oxide pigment, and a concealed film thickness of 25 µm or less per 100 parts by weight of the thermosetting resin composition. Using,

(2) As the final coating paint (C), a solid color paint, a metal paint or an interference paint is used, and

(3) After heat-curing the coating film of the said intermediate coating paint (B), the said final coating paint (C) is coated.

It provides a method of forming a multilayer coating film characterized in that. By this method, the intermediate coating film can be thinned and a multilayer coating film excellent in smoothness can be formed.

Description

Formation method of multilayer coating {METHOD OF FORMING MULTIPLE-LAYERED COATING FILM}

Background Art A method of forming a multilayer coating by sequentially coating a primary coating (such as electrodeposition coating), an intermediate coating and a final coating is known. However, in the case of the intermediate coating paint, in order to conceal the lower layer and maintain the coating performance, it is usually necessary to coat the film with a thickness of 30 µm or more (as a cured coating film). For this purpose, it is desired to reduce the coating cost of the entire multilayer coating film by making the intermediate coating film thin without reducing concealment or coating performance.

The present inventors continue to earnest research in order to solve these problems, as a result, in the process of sequentially coating the primary coating, intermediate coating and final coating paint, all the components of aluminum powder and titanium oxide pigment as intermediate coating paint By using the thermosetting paint containing and coating the final coating after curing of the intermediate coating film, the underlying coating of the intermediate coating film is improved and the chipping resistance is also improved to make the intermediate coating film thin. Furthermore, the present inventors have found that the smoothness of the final coating drawing is also improved, and thus, the present invention has been completed.

In the present invention, when forming a multilayer coating by sequentially coating the primary coating, the intermediate coating and the final coating, by using an intermediate coating of a specific pigment composition, and after curing the coating, the final coating is coated. In addition, the present invention relates to a method of forming a multilayer coating film which can thin the intermediate coating film without degrading the coating performance and also has excellent smoothness.

The present invention provides a method of forming a multilayer coating film by sequentially coating a primary coating (A), an intermediate coating (B) and a final coating (C) on a substrate,

(1) As the intermediate coating paint (B), a liquid thermosetting paint containing 0.1 to 30 parts by weight of aluminum powder and 1 to 200 parts by weight of titanium oxide pigment and a concealed film thickness of 25 μm or less per 100 parts by weight of the thermosetting resin composition. Using,

(2) As the final coating paint (C), a solid color paint, a metal paint or an interference paint is used, and

(3) After heat-curing the coating film of the said intermediate coating paint (B), the said final coating paint (C) is coated.

It is to provide a method of forming a multilayer coating film characterized in that.

Hereinafter, the multilayer coating film forming method of the present invention (hereinafter referred to as "the present method") will be described in more detail.

Primary coating paint (A) :

The primary coating paint (A) is used to impart rust resistance, adhesion, etc. by coating directly on a substrate made of metal or plastic, that is, a coated object. Any primary coating can be used. As the to-be-painted article to which the above primary coating can be applied, an automobile exterior plate is particularly preferable. In addition, it is preferable that the to-be-coated thing normally performs rust removal, washing | cleaning, chemical conversion treatment, etc. suitably previously.

When the to-be-coated object is a metal back surface or an electroconductive surface, a cationic electrodeposition paint is preferable as a primary coating paint. As the cationic electrodeposition paint, one known per se, which is obtained by mixing a salt solution or an aqueous dispersion of a cationic polymer compound with a crosslinking agent, a pigment or various additives, can be used, and the kind thereof is not particularly limited. As a cationic high molecular compound, what introduce | transduced cationic groups, such as an amino group, into the acrylic resin or epoxy resin which has a crosslinkable functional group, for example, can be water-soluble or water-oxidized by neutralizing with an organic acid, an inorganic acid, etc. As a crosslinking agent for hardening these high molecular compounds, a floc polyisocyanate compound, an alicyclic epoxy resin, etc. can be used preferably.

Electrodeposition coating is performed by immersing metal coating materials, such as automobile exterior parts or bumpers, as a cathode in the bath of the said cationic electrodeposition coating, and energizing them with the conditions of a conventional method between the anodes and precipitating the coating on the coating objects. Can be. The film thickness of the formed electrodeposition coating film is preferably within the range of 10 to 40 µm based on the cured paint, and the coating film can be crosslinked and cured by heating at about 140 to 220 ° C for about 10 to 40 minutes. In the present method, it is preferable to apply the intermediate coating paint after curing the electrodeposition coating film, but in some cases, the intermediate coating paint may be applied in an uncured state.

Intermediate coating paint (B) :

In the present method, a liquid thermosetting paint containing 0.1 to 30 parts by weight of aluminum powder and 1 to 200 parts by weight of titanium oxide pigment and a concealed film thickness of 25 μm or less per 100 parts by weight of the thermosetting resin composition is used as the intermediate coating paint (B). do.

By using aluminum powder and titanium oxide pigment together in the intermediate coating paint (B), the hiding power of the coating film is increased, and even a thin film of 25 μm or less, especially 10 to 25 μm, can be sufficiently concealed (primary coating drawing) in the cured coating film. This makes it possible to achieve thinning of the intermediate coating film.

In such an intermediate coating paint (B), the thermosetting resin composition used as the vehicle component basically consists of a base resin and a crosslinking agent or a self-crosslinking curable resin, wherein the base resin is, for example, a hydroxyl group and an epoxy group. And acrylic resins having two or more crosslinking functional groups such as isocyanate groups and carboxyl groups in one molecule, polyester resins, alkyd resins, and the like, and for example, amino resins such as melamine resins or urea resins. , Polyisocyanate compounds, carboxyl group-containing compounds, etc., which may be flocculated, may be used. Examples of the self-crosslinking curable resin include resins containing two or more alkoxysilane groups in one molecule, resins containing a carboxyl group and a hydroxyl group in one molecule, resins containing an isocyanate group which may be flocculated in one molecule, and the like. These resins are based on vinyl resin, acrylic resin, polyester resin, urethane resin, etc., for example.

As the aluminum powder to be blended into the intermediate coating paint (B), an aluminum powder having a mean particle size of generally 40 μm or less, preferably 10 μm or less and more preferably 3 to 7 μm is suitable, and in particular, an average particle size When the fine powder of 10 micrometers or less is used, the intermediate coating film formed itself will not be shiny (brightness). Here, "average particle diameter" means the diameter measured by the laser diffraction scattering method (LA-500). The main component of this aluminum powder is metal aluminum, but the surface thereof may be treated with a silane coupling agent or the like.

As the titanium oxide pigment blended with the intermediate coating paint (B) according to the present method, one known per se as a coating pigment can be used, and the average particle diameter thereof is preferably 5 µm or less. In addition, the surface of the titanium oxide pigment may be treated with alumina or silica.

The compounding amount of the aluminum powder and the titanium oxide pigment is in the range of 0.1 to 30 parts by weight, preferably 0.5 to 20 parts by weight, more preferably 1 to 7 parts by weight, per 100 parts by weight (as a solid) of the thermosetting resin composition. The titanium oxide pigment may be in the range of 1 to 200 parts by weight, preferably 40 to 160 parts by weight, more preferably 80 to 120 parts by weight. The aluminum powder is preferably used in the range of 1 to 15 parts by weight, preferably 1.5 to 10 parts by weight, more preferably 2 to 7 parts by weight, per 100 parts by weight of the titanium oxide pigment.

It is essential that the intermediate coating paint (B) used in the present method contains both an aluminum powder and a titanium oxide pigment, and the total compounding amount of these pigments is 25 μm of the concealed film thickness of the coating film formed using the paint (B). In particular, the amount can be set to 10 to 25 µm or less (as a cured coating film). Here, the "concealed film thickness" is the smallest coating film in which the base (skin) color penetrates the coating film and cannot be recognized. Specifically, the black and white film is transmitted through the coating film coated on the black and white checkered plate. The minimum film thickness that cannot be determined. In this method, by mixing both the aluminum powder and the titanium oxide pigment in a specific amount by combining the intermediate coating paint (B), the concealed film thickness of the coating film can be thinned to 25 µm or less. That is, the background color can be sufficiently concealed even in a thin film of 25 µm or less. If either component is lacking, it is difficult to conceal with such a thin film.

The intermediate coating paint (B) can be prepared by mixing and dispersing the components of the thermosetting resin composition, the aluminum powder and the titanium oxide pigment in a solvent such as an organic solvent and / or water, and further, the aluminum powder and oxidation as necessary. Colored pigments other than titanium pigments, extender pigments, sedimentation inhibitors and the like may be suitably blended.

The intermediate coating paint (B) is 25 μm or less, in particular 10 to 25 μm, based on the cured coating film by a method such as electrostatic coating, air spray, airless spray, or the like on a hardened or uncured primary coating. It is preferable to coat with the film thickness within a range.

In this method, after heat-hardening the coating film of intermediate coating paint (B), the final coating paint (C) described below is coated. The heat hardening of the coating film in an intermediate coating paint (B) can be performed, for example by heating the said coating film at the temperature of about 140 to about 200 degreeC for about 10 to about 40 minutes.

Final coating paint (C) :

According to the present invention, a solid color (colored) paint (C-1), a metal paint (C-2) or an interference paint (C-3) is coated on the cured drawing of the intermediate coating paint (B) as a final coating paint. . It is preferable that all of these final coating materials are thermosetting.

First, as the solid color paint (C-1), preferably as a liquid thermosetting paint mainly composed of a thermosetting resin composition and a coloring pigment, those which do not substantially contain a metal pigment or an interference color pigment are used.

The thermosetting resin composition used as the coloring paint (C-1) basically consists of a base resin, a crosslinking agent, or a self-crosslinking curable resin, wherein the base resin is, for example, a hydroxyl group, an epoxy group, an isocyanate group, a carboxyl group, and the like. The acrylic resin, polyester resin, alkyd resin etc. which have 2 or more crosslinkable functional groups in 1 molecule can be mentioned, As a crosslinking agent, For example, melamine resin, amino resin, such as urea resin, and polyisocyanate which may be flocculated And carboxyl group-containing compounds. As the self-crosslinking curable resin, for example, a resin containing two or more alkoxysilane groups in one molecule, a resin containing a carboxyl group and a hydroxyl group in one molecule, and a resin containing an isocyanate group which may be fused with a hydroxyl group in one molecule These resins are based on vinyl resin, acrylic resin, polyester resin, urethane resin, etc., for example.

The coloring pigment which can be mix | blended with solid colorado pigment (C-1) is a component for giving solid-color tone to the multilayer coating film formed by the method of this invention substantially without containing a metallic pigment or an interference pigment, and Organic or inorganic coloring pigments for paint can be used. Specific examples include inorganic pigments such as titanium oxide, zinc oxide, carbon black, cadmium red, molybdenum red, chrome yellow, chromium oxide, purcyanogen blue, and cobalt blue; Organic pigments such as azo pigments, phthalocyanine pigments, quinacridon pigments, isoindolin pigments, insanthrene pigments and perylene pigments. It is preferable that these pigments generally have an average particle diameter of 5 micrometers or less.

The blending amount of these pigments may be arbitrarily selected according to the coloring power or purpose of the pigment itself, but is generally in the range of 0.5 to 200 parts by weight, preferably 1 to 150 parts by weight per 100 parts by weight of the thermosetting resin composition. The concealed film thickness of a coating film can be set to 50 micrometers or less, especially 40 micrometers or less with a cured coating film.

The solid color paint (C-1) can be prepared by mixing and dispersing the above components with a solvent such as an organic solvent and / or water, and the paint can be suitably blended with a extender pigment, a sedimentation inhibitor, or the like as necessary.

As the metal coating (C-2), a liquid thermosetting coating containing preferably the thermosetting resin composition and the metal pigment as described above with respect to the solid color coating material (C-1) can be used. The metal pigment blended with this metal paint is a particle pigment of a flaky metal or metal oxide having a shiny brightness, and specific examples thereof include aluminum powder, mica iron oxide, and the like. . The flaky pigment particles may generally have an average particle diameter in the range of 10 µm or more, preferably 10 to 50 µm, more preferably 15 to 40 µm. The compounding amount of this metal pigment is generally in the range of 0.1 to 20 parts by weight, preferably 3 to 10 parts by weight per 100 parts by weight of the thermosetting resin composition, and the concealed film thickness of the formed coating film is 50 µm or less as a cured coating film, In particular, it can be set as the quantity used as 30 micrometers or less.

The metal paint (C-2) can be prepared by mixing and dispersing the above components with a solvent such as an organic solvent and / or water, and the paint can be suitably blended with extender pigments, coloring pigments, anti-sedimentation agents and the like as necessary. have. Since the metal coating (C-2) contains a metal pigment having a relatively large particle size, the coating film of the metal coating itself exhibits a shiny glossiness.

As the interference-like paint (C-3), preferably, the thermosetting resin composition as described above with respect to the solid color coating material (C-1) and the liquid thermosetting paint mainly containing the interference pigment can be used. As the interference pigment to be incorporated into the interference-like paint, flaky mica and so-called interference mica coated on the surface with a metal oxide such as titanium oxide or iron oxide are particularly preferable. In this interference mica, the coating thickness of the metal oxide is preferably 200 nm or more based on the optical thickness and 80 nm or more based on the geometric thickness, and when the film thickness becomes thinner than this, in general, the interference effect of light It is not preferable because it is lowered. The interference pigment may have an average particle diameter in the range of usually 10 μm or more, preferably 10 to 50 μm, more preferably 15 to 40 μm.

The blending amount of the interference pigment may be in the range of 1 to 100 parts by weight, preferably 5 to 50 parts by weight, per 100 parts by weight of the thermosetting resin composition.

The interference-like paint (C-3) may be prepared by mixing and dispersing the above components with a solvent such as an organic solvent and / or water, and the like, which may include coloring pigments, metal pigments, sieving pigments, and antisettling agents, if necessary. It can mix | blend suitably.

The final coating paint (C) described above is a method of electrostatic coating, air spray, airless spray, etc., on the heat-cured intermediate coating, and is in the range of 10 to 60 μm, particularly 20 to 35 μm, based on the cured coating. It is preferable to paint in thickness.

The coating film of the final coating paint (C) can be cured by heating at a temperature of about 120 to about 180 ° C for 10 to 40 minutes, for example.

Clear Paint (D) :

In this method, the clear paint (D) can be coated as needed on the drawing of the final coating paint (C) of the multilayer coating film formed as described above. The clear paint (D) can be painted on the drawing of the final coating paint (C) in the cured or uncured state formed as described above.

As clear paint (D), Preferably it is a thermosetting resin composition and a solvent, and a coloring pigment, a metal pigment, an interference pigment, an ultraviolet absorber, and other paint additives in the grade which does not impair the transparency of a coating film as needed. It is possible to use a liquid coating obtained by blending.

The thermosetting resin composition basically consists of a base resin and a crosslinking agent, or a self-crosslinking curable resin, wherein the base resin includes, for example, a crosslinkable functional group such as a hydroxyl group, an epoxy group, an isocyanate group, or a carboxyl group in one molecule. Acrylic resins, polyester resins, alkyd resins, urethane resins and the like which have two or more. Examples of the crosslinking agent include melamine resins, urea resins, polyisocyanate compounds which may be flocculated, and carboxyl group-containing compounds. have. As the self-crosslinking curable resin, for example, a resin containing two or more alkoxysilane groups in one molecule, a resin containing a carboxyl group and a hydroxyl group in one molecule, a resin containing a hydroxyl group and a isocyanate group which may be flocculated in one molecule, etc. These resins are based on vinyl resin, acrylic resin, polyester resin, urethane resin, etc., for example. An organic solvent and / or water can be used as a solvent, A clear coating (D) can be prepared by melt | dissolving or disperse | distributing the said thermosetting resin composition and other components in these solvents.

The clear paint (D) may be coated on the drawings of the uncured or cured final coating (C) formed as described above by electrostatic coating, air spray, airless spray, etc., and the film thickness is based on the cured coating film. It is preferable to set it as 10-60 micrometers, especially the range of 20-50 micrometers. The coating film itself of the said clear paint (D) can crosslink-cure for 10 to 40 minutes at the temperature of about 120-180 degreeC.

According to the multilayer coating film formation method of this invention described above, the effect as described below can be acquired, for example.

(1) Since the thickness of the intermediate coating film can be made thinner (usually 30 µm or more) than the conventional one (usually 30 µm or more), the cost of the entire multilayer coating film can be reduced.

(2) Since the final coating paint is coated after heat curing the intermediate coating coating, the smoothness of the final coating coating is excellent.

(3) Since the intermediate coating paint is excellent in concealment of the base, even when coated with a thin film, the color stability of the final coating film is good, and the color coating of the final coating film can be freely changed according to the purpose.

(4) The formed multilayer coating film is excellent in chipping resistance.

Hereinafter, the method of the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to Examples.

I. sample

(1) Cationic electrodeposition paint (A)

"Electron 9400 HB" (Kansai Paint Co., Ltd. make, brand name, epoxy resin polyamine floc polyisocyanate compound type)

(2) Intermediate Coating Paint (B)

An organic solvent coating material comprising a polyester resin, melamine resin, fine aluminum powder and titanium oxide pigment in a ratio shown in Table 1 below. The compounding quantity of each component of Table 1 is a ratio of weight solid content.

Intermediate Coating Paint (B) B-1 B-2 B-3 B-4 B-5 Polyester resin (* 1) 65 70 75 70 70 Melamine Resin (* 2) 35 30 25 30 30 Fine Aluminum Powder (* 3) 5 2 2 - 2 Titanium Oxide Pigment (* 4) 120 100 80 80 - Iron Oxide Pigment (Red) (* 5) 2 2 2 2 2 Concealed Film Thickness (㎛) (* 6) 11 13 15 100 150

(* 1): Polyester resin of a phthalic anhydride and hexane hydrophthalic anhydride type (number average molecular weight about 4000, hydroxyl value 82, acid value 7).

(* 2): Placenta 28-60 (Mitsui Toatsuka Chemical Co., Ltd. product name)

(* 3): K-9800 (manufactured by Asahi Chemical Co., Ltd., brand name), average particle size 5 ~ 6 ㎛

(* 4): Titanium JR701 (made by Daigo Cugo Co., Ltd.), average particle size 0.3 ~ 0.6 ㎛

(* 5): KNO-W Sangatetsu (manufactured by Toda Kogyo KK, trade name), average particle diameter 0.2 ~ 0.5 ㎛ (red

Solid color pigment)

(* 6): Black and white discrimination can be made with the naked eye through a coating film coated on a checkered black and white plate.

The minimum film thickness (μm) without was measured.

(3) Final Coating Paint (C)

An organic solvent type paint formed by blending acrylic resin, melamine resin and colored pigment or metal pigment as shown in the following Table 2. The compounding quantity of each component of Table 2 is a ratio of weight solid content.

Final Coating Paint (C) C-1 C-2 C-3 Acrylic resin (* 7) 65 70 75 Melamine resin (* 8) 35 30 25 Titanium White Pigment (* 9) 80 - - Carbon Black (* 10) 0.2 - - Interference Pigment (* 11) - 9 9 Concealed Film Thickness (㎛) (* 6) 100 < 100 < 100 <

(* 7): Methyl methacrylate acrylic resin, having a number average molecular weight of about 2000, a hydroxyl value of 70, and an acid value of 8.

(* 8): Placenta 28-60 (Mitsuido Atsuga Chemical Co., Ltd. product name)

(* 9): Titanium CR93 (manufactured by Ishihara Sangyo, trade name)

(* 10): Carbon FW200 (manufactured by DEGUSSA, trade name)

(* 11): Xteria highlight light blue (manufactured by Marusha, trade name, average particle diameter: 14 ~ 18 ㎛)

(5) Clear Paint (D)

"Magicron clear" (Kansai Paint Co., Ltd. make, brand name, an acrylic resin, melamine resin type, an organic solvent type).

II. Examples and Comparative Examples

Using the sample, the coating was carried out in accordance with the coating step shown in Table 3, and cured by heating to form a multilayer coating film. Table 3 also shows the performance test results of the multilayer coating.

Example Comparative example One 2 3 One 2 3 Electrodeposition paint (A) Heating condition 170 ° C., 30 minutes Intermediate Coating Paint B-1 B-2 B-3 B-4 B-5 B-1 Drying condition 140 ° C, 30 minutes 5 minutes at room temperature Final coating paint C-1 C-2 C-3 C-1 C-2 C-1 Drying condition 140 ° C, 30 minutes 5 minutes at room temperature 140 ° C, 30 minutes 5 minutes at room temperature 140 ° C, 30 minutes Clear paint - D - D - Heating condition - 140 ℃, 30 minutes - 140 ° C, 30 minutes - Performance test result Smoothness ○ ○ ○ ○ ○ × Appearance ○ ○ ○ × × △ Metallic - ○ ○ - △ - Chipping ○ ○ ○ △ △ ○

The deionized and zinc phosphate-treated steel sheets were electrodeposited with a cationic electrodeposition coating (A) so as to have a film thickness of 20 μm by a conventional method, and heated at 170 ° C. for 30 minutes to cure the coating film. Intermediate coating paints (B-1) to (B-5) were coated on the electrodeposition drawing so as to have a film thickness of 25 μm, and in Examples 1 to 3 and Comparative Examples 1 and 2, the intermediate coating paint was heated at 140 ° C. for 30 minutes. Was cured, while Comparative Example 3 was left to stand at room temperature for 5 minutes. After that, the final coating paints (C-1) to (C-3) were applied to the intermediate coating drawing using a minibell-type electrostatic coating machine, with a discharge amount of 150 kPa, a rotation speed of 50000 rpm, and a shaving pressure of 1 kg /. 2 cm 2, gun distance 30 cm, booth temperature 20 ° C, booth humidity 75%. The coating film thickness was 15-25 micrometers. After leaving the final coating paint film for 5 minutes in the booth, in Example 1 and Comparative Examples 1 and 3, the coating film of the final coating paint (C) was cured by heating at 140 ° C for 30 minutes. On the other hand, in Examples 2, 3 and Comparative Example 2, the discharge amount of 300 kPa and the number of revolutions of 40000 were obtained by using a minibell-type electrostatic coating machine for the clear coating (D) in the drawing of the uncured final coating (C) The coating was carried out at rpm, a shaping pressure of 5 kg / cm 2, a total distance of 30 cm, a booth temperature of 20 ° C., and a booth humidity of 75%. The coating film thickness was 45-50 micrometers. After coating, the mixture was left at room temperature for 3 minutes, and then heated at 140 ° C. for 30 minutes using a hot air circulation drying furnace, and the two-layer coating film formed of the final coating paint (C) and clear coating (D) was simultaneously cured.

Coating performance test method and evaluation criteria are as follows.

Smoothness : Visual evaluation. (Circle): good, (triangle | delta): surface becomes a little rough, x: surface is remarkably

Roughness.

Finishing appearance : Visually assess color bleeding and concealment. ○ is good, △ is slightly good, ×

Is bad.

Metallic texture : Visual evaluation of metal smears. (Circle) is good, (triangle | delta) is a little good, and x is bad.

Chipping resistance : No. 7 crushed stone was used as a testing machine using Glaberometa (produced by Q Panel).

3 g / cm 2 of pneumatic pressure at 20 ° C. at 500 ° to the drawing

Blown into the film to impact it. Then, the adhesive tape on the drawing

After adhering and abruptly peeling it off,

Investigate the state of membrane peeling.

(Circle): No or almost no peeling of a film in a damaged periphery,

△: coating film peeling is clearly seen at damaged periphery,

X: The peeling of a coating film is remarkable at the damaged peripheral part.

Claims (13)

In the method of forming a multilayer coating film by sequentially coating a primary coating (A), an intermediate coating (B) and a final coating (C) on a substrate, (1) The intermediate coating paint (B), which contains 0.1 to 30 parts by weight of aluminum powder and 1 to 200 parts by weight of titanium oxide pigment per 100 parts by weight of the thermosetting resin composition and has a concealed film thickness of 10 to 25 μm. Using thermosetting paint, (2) As the final coating paint (C), a solid color paint, a metal paint or an interference paint is used, and (3) After heat-curing the coating film of the said intermediate coating paint (B), the said final coating paint (C) is coated. Forming a multilayer coating film, characterized in that. The method according to claim 1, wherein the aluminum powder contained in the intermediate coating paint (B) is an average particle diameter of 3 to 40 µm. The method according to claim 2, wherein the aluminum powder has an average particle diameter of 3-10 mu m. The method according to claim 1, wherein the titanium oxide pigment contained in the intermediate coating paint (B) is an average particle diameter of 0.3 to 5 µm. The method according to claim 1, wherein the intermediate coating paint (B) contains 1 to 7 parts by weight of aluminum powder and 80 to 120 parts by weight of titanium oxide pigment per 100 parts by weight of the thermosetting resin composition. The method according to claim 1, wherein the intermediate coating paint (B) contains 1 to 15 parts by weight of aluminum powder per 100 parts by weight of titanium oxide pigment. delete The method according to claim 1, wherein the intermediate coating paint (B) is coated so as to have a thickness within the range of 10 to 25 µm based on the cured coating film. The method according to claim 1, wherein the coating film of the intermediate coating paint (B) is cured by heating to a temperature of 140 to 200 ° C. The method according to claim 1, wherein the final coating paint (C) is coated so as to have a film thickness within the range of 10 to 60 µm based on the cured coating film. The method according to claim 1, wherein the coating film of the final coating paint (C) is cured by heating to a temperature of 120 to 180 deg. The method according to claim 1, wherein the primary coating paint (A) is a cationic electrodeposition paint. The method of claim 1, wherein the clear coating (D) is further coated on the drawing of the final coating (C).