JPH10219138A - Rustproof film and its coating method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rustproof film which has good hydrophilicity, air permeability, and dryability and is excellent in corrosion resistance. SOLUTION: This rustproof film comprises an undercoating layer and a topcoating layer formed on the surface of a substrate mainly comprising iron. The undercoating layer is formed by applying at least one metal selected from among zinc, tin, lead, aluminum, chromium, nickel, and copper by melt plating, electrical plating, penetration plating, metal spraying or chemical plating or is formed by applying a coating material contg. at least one rustproof pigment selected from among a zinc dust, zinc chromate, red lead oxide, basic lead chromate, strontium chromate, and lead chromate. The topcoating layer is formed by applying a coating compsn. mainly comprising 100 pts.wt. inorg. filler, 5-30 pts.wt. (solid basis) binder, and 10-100 pts.wt. hydrophilic org. solvent and/or water and curing by normal-temp. drying or low-temp heat drying.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rust preventive film and a method of coating the same, and more particularly, to a method of coating a specific coating composition after coating the surface of iron with a metal less noble than iron or a metal having better corrosion resistance. The present invention relates to a rust-preventive film having good hydrophilicity, air permeability and drying properties and excellent corrosion resistance, which is obtained by applying a substance and drying at room temperature or under low-temperature heating, and a coating method thereof.

[0002]

2. Description of the Related Art Conventionally, as a method for preventing rust of iron, in order to prevent an anodic reaction (formation of Fe ²⁺ ) and a cathodic reaction (formation of OH ⁻ ) occurring on the surface of iron, it has been proposed to use iron. Lower the electrode potential, coat the iron surface to stop the movement of ions, or block the permeation of water and oxygen,
And the like. As a specific example of the rust prevention method, coating with a metal that is lower than iron or a metal having better corrosion resistance, a coating processing method for shielding the environment, and the like are used. However, in the case of only the former metal coating, there is a problem in the durability and corrosion resistance of the metal, and in the case of the latter blocking coating,
There are problems with the coating of the edge portion, pinhole-lessness, durability at the time of occurrence of scratches, workability and cost.

[0003]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and provides a rust preventive film having good hydrophilicity, air permeability, drying property, and excellent corrosion resistance, and a method of coating the same. The purpose is to do.

[0004]

According to the present invention, at least one kind of metal selected from the group consisting of zinc, tin, lead, aluminum, chromium, nickel and copper is provided on the surface of a substrate mainly composed of iron. Coated by hot-dip plating, electroplating, penetration plating, thermal spraying or chemical plating, or selected from the group of zinc dust, zinc chromate, lead red, basic lead chromate, strontium chromate and lead chromate An undercoat layer coated with a paint containing at least one type of rust-preventive pigment, and a top coat formed by applying a coating composition containing the following components (a) to (c) as main components and curing by drying at room temperature or at low temperature. And a coating method therefor. (A) 100 parts by weight of inorganic filler (b) 5 to 30 parts by weight of binder in terms of solid content (c) 10 to 100 parts by weight of water and / or hydrophilic organic solvent

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The rust-preventive film of the present invention comprises a corrosion-resistant metal layer covering the surface of iron, or a paint film containing a rust-preventive pigment (undercoat layer), and a hydrophilic and breathable film for protecting it. It is characterized in that it is formed from a protective film (overcoat layer). Undercoating layer The undercoating layer is a rust-preventive pigment mainly composed of a metal or a metal compound having a lower ionization tendency than iron, or a plating layer of a metal having good corrosion resistance is formed on the surface of a base material mainly composed of iron. It is formed from a paint layer containing: here,
Examples of the base material mainly composed of iron include a flat plate, a corrugated plate, a steel material, and a bar made of iron such as pure iron, steel, and cast iron, and the material and shape thereof are not particularly limited.

The metal used for the plating is at least one selected from the group consisting of zinc, tin, lead, aluminum, chromium, nickel and copper.
Tin and chromium are preferred. Among the plating methods, the hot-dip plating method is a method in which iron is immersed in a bath of a molten metal such as zinc, tin, aluminum, or a lead-tin alloy. The electroplating method is a method of electrolyzing and depositing metal cations such as zinc, tin, copper, nickel, and chromium in the aqueous solution containing metal ions on the surface of iron used as a cathode in the aqueous solution. The infiltration plating method is used to
In this method, an alloy with chromium, aluminum, zinc, or the like is formed on the surface of iron by infiltration. The thermal spraying method is a method in which a metal such as zinc, aluminum, tin, or lead melted by an arc or the like is sprayed on the surface of iron to form a thick film. The chemical plating method is a method of electrolessly depositing metals such as nickel, chromium, and copper on the surface of iron by using a reducing agent from an aqueous solution containing the metal ions. Specific examples of the undercoat layer formed by these plating methods include a tin plate and a tin plate. The thickness of the undercoat layer by these plating methods is not particularly limited,
Usually, in the case of the zinc hot-dip plating method, the zinc deposition amount is 50 to 3
00g / m ² , in the case of the tin hot-dip plating method, the tin adhesion amount is 20 to 30 g / m ² , in the case of the zinc electroplating method, the zinc adhesion amount is 10 to 55 g / m ² , and in the case of the tin electroplating method,
The tin deposit is 5-20 g / m ² .

When an undercoat layer is formed from a paint containing a rust preventive pigment, examples of the rust preventive pigment include zinc dust, zinc chromate, lead red, basic lead chromate, strontium chromate and lead chromate. At least one selected from the group consisting of: As the coating material, a synthetic resin, an alkoxysilane, a silicate, or the like as a binder, zinc powder,
Paints containing anticorrosive pigments such as zinc chromate, lead red, basic lead chromate, strontium clot, zinc chromate, and lead suboxide are exemplified. The ratio of the rust-preventive pigment in the coating material is usually 10 to 90% by weight, preferably 50 to 85% by weight in terms of solid content. Specific examples of the paint containing the rust-preventive pigment include Ceramica G1-720Z (manufactured by Nissin Laboratories Co., Ltd.) and Ceramica MSA-800Z (manufactured by Nissin Labs. Co., Ltd.). The thickness of the undercoat layer made of a paint containing these rust preventive pigments is not particularly limited, but is usually 5 to 200 μm, preferably 15 to 160 μm in dry film thickness.
μm. When the undercoat layer is formed with the rust-preventive pigment-containing paint, the curing conditions are usually room temperature drying for 1 to 24 hours, or 100 to 150 ° C for 10 to 10 hours.
60 minutes.

Overcoat layer The overcoat layer of the present invention protects the undercoat layer and has hydrophilicity,
It has good breathability, good drying properties, moderately ionizes the metal or rust preventive pigment of the undercoat layer, and plays the role of a protective film that does not form a local battery on the base material. Essential to get. This overcoat layer is formed by the above (a)
The composition is formed by applying a coating composition mainly composed of the components (c) to (c) and curing the coating composition by drying at room temperature or drying by heating at low temperature.

(A) Inorganic filler; (a) The inorganic filler is necessary for forming a hydrophilic thick film, and further imparts coloring to the coating film and imparts various functions to the coating film, for example, It imparts heat insulation, conductivity, heat radiation, abrasion resistance, antibacterial properties, etc. to the coating film. (A) As the inorganic filler, those having an average particle diameter or an average length of preferably 0.02 to 50 μm, more preferably 0.2 to 20 μm are used. The average particle size is
If the thickness is less than 0.02 μm, a required film thickness cannot be obtained,
The functionality of the inorganic filler cannot be exhibited or the production cost becomes too high. On the other hand, if it exceeds 50 μm, the coating film becomes rough or the adhesion decreases. The inorganic filler (a) is preferably a water-insoluble, particulate or fibrous material, and includes at least one selected from the group consisting of inorganic extender, functional pigment, inorganic pigment and metal powder. be able to.

(A) Specific examples of the inorganic filler include commercially available silica, alumina, kaolin, zircon, tin oxide, mullite, zeolite, talc, perlite, carbon, potassium titanate whisker, silicon carbide, silicon nitride, and titanium nitride. , Iron, manganese, cobalt, chromium, nickel and other oxides and carbon or cobalt and aluminum, iron and manganese and other synthetic oxides, or iron,
Examples include inorganic pigments such as three kinds of synthetic oxides such as copper and chromium, and metal powders such as zinc, nickel, stainless steel, aluminum, and tin, but are not limited thereto.

(B) a binder; (b) a binder functions as an adhesive for a coating film obtained in a coating composition, and is composed of silicate, colloidal silica, colloidal alumina, and synthetic resin. At least one selected from the group of Among them, the silicate has a property that a silanol group (Si-OH) is condensed by dehydration to form a three-dimensional structure. In the coating composition of the present invention, the silicate serves as a binder for a coating film obtained. To fulfill. Taking lithium silicate as an example, the structural formula of a silicate is as follows. (In the formula, p represents the number of repeating units.)

Taking a specific example of silicate as an anhydrous salt,
Lithium silicate, sodium silicate, potassium silicate,
Cesium silicate, amine silicate and the like can be mentioned. Among them, examples of the amine silicate include liquid silicates such as dimethylethanol ammonium silicate, monomethyl tripropanol ammonium silicate, dimethyl dipropanol ammonium silicate, and monoethyl tripropanol ammonium silicate. These silicate amines are prepared by adding a quaternary ammonium hydroxide to an activated silica solution obtained by contacting a diluted water glass with a hydrogen-type cation exchange resin and concentrating the solution to a predetermined concentration, or It can be easily obtained by a method of reacting an ammonium hydroxide with a silica hydrosol. Here, the quaternary ammonium hydroxide is usually obtained by a method of adding an alkylene oxide to ammonia or amines, or a method of deionizing a quaternary amine salt with an anion exchange resin.
Products containing a small amount of tertiary, secondary or primary amines in the product can also be used, and silicate amines obtained using the same can also be used in the composition of the present invention. can do. Note that in the silicic acid amine, it is preferred that SiO ₂ is contained in the range of 5 to 70 wt%.

Specific examples of the above-mentioned amine silicate include amine silicate CAS 25 manufactured by Nissan Chemical Industries, Ltd.
[SiO ₂ content = 25 to 26% by weight, specific gravity (20 ° C.)
= 1.19, viscosity (cps) = 3.0, pH = 11.
0], the same cast 40 [SiO ₂ content = 40 to 41% by weight, specific gravity (20 ° C.) = 1.32, viscosity (cps) = 1
0.0, pH = 11.2], NS manufactured by Nippon Laboratories Co., Ltd.
-20 (SiO ₂ concentration = about 20% by weight). These silicates can be used alone or in combination of two or more.

Among the binders (b), colloidal silica is a single-particle polymer silica sol having a silanol group on the surface of a siloxane bond, and when gelled, this polymer single particle becomes a silanol group. To form a siloxane-bonded chain to form a three-dimensional siloxane-bonded network while multimerizing into pure silica gel. Colloidal silica is used as a binder in coating compositions. In addition, when used alone, the colloidal silica can form only a film having a thickness of 0.1 to 0.3 μm and has little use. However, due to the combination with (a) an inorganic filler, the colloidal silica has a thickness of 1 to 300 μm. A coating film having a thickness can be formed. Specific examples of the colloidal silica include Nissan Chemical Industries, Ltd., Snowtex Series, Catalyst Kasei Kogyo Co., Ltd., Cataloid Series, Asahi Denka Kogyo Co., Ltd., Adelite Series.

Colloidal alumina is an alumina sol having a pH of 2.5 to 6 using water and / or lower alcohols as a dispersion medium, containing 5 to 25% by weight of alumina, and as a stabilizer such as nitric acid, hydrochloric acid and acetic acid. An acid was used. As the above-mentioned colloidal alumina, a granular or feather-like material having an average particle diameter or an average thickness of 5 to 50 mμ is preferable.

Among the binders (b), the synthetic resin serves as an adhesion aid for the resulting coating film, and (a)
It functions as a buffer for the inorganic filler, prevents cracking and peeling of the coating film, and improves impact resistance. Specific examples of the synthetic resin include an emulsion resin and a water-soluble resin such as an acrylic resin, a vinyl acetate resin, a melamine resin, a phenol resin, a polybutadiene resin, an alkyd resin, and a silicone resin. For example, the above synthetic resin is mixed well with water and / or a hydrophilic organic solvent and dried to form a water-insoluble transparent or translucent film. In the above coating composition, (a) an inorganic filler It is used for bonding.

The above-mentioned binder (b) is contained in the coating composition.
(A) 100 parts by weight of the inorganic filler, in terms of solid content,
It is 5 to 30 parts by weight, preferably 10 to 25 parts by weight.
If the amount is less than 5 parts by weight, the adhesive strength is weak, while if it exceeds 30 parts by weight, the covering power is too large for component (a), cracks occur, hydrophilicity and air permeability are impaired, which is not preferable.

(C) water and / or a hydrophilic organic solvent;
Water used in the coating composition is an essential component for adjusting the viscosity of the composition, adjusting the pot life, and further dispersing the inorganic filler. As this water, tap water, distilled water, and ion-exchanged water can be used. The water also includes water contained in the above silicate, colloidal silica, colloidal alumina emulsion type or water-soluble type synthetic resin.

On the other hand, the hydrophilic organic solvent used in the above-mentioned coating composition is an organic solvent compatible with water, and comprises an agent for adjusting the solid content and viscosity of the composition, and an agent for adjusting the drying speed.
Alternatively, it is used as an antifreeze solution.

The hydrophilic organic solvent includes alcohols, glycols, esters, ethers, ketones and the like. Examples of the alcohols include aliphatic alcohols having 1 to 8 carbon atoms, such as methanol, ethanol, n-propanol, i-propanol, n-butanol, and sec-
Butanol, t-butanol, methyl carbitol and the like. Examples of the glycols include ethylene glycol, propylene glycol, diethylene glycol, and the like. As the esters, formic acid, acetic acid, propionic acid and the like of the alcohols and glycols, specifically methyl formate, ethyl formate,
Examples thereof include butyl formate, methyl acetate, ethyl acetate, butyl acetate, and ethyl propionate.

Examples of the ethers include the alkyl ethers of the alcohols and glycols, such as dimethyl ether, diethyl ether, dibutyl ether, methyl ethyl ether, ethyl butyl ether, ethylene glycol monobutyl ether, ethylene glycol monoethyl ether acetate and propylene. Glycol monoethyl ether and the like. Ketones include acetone, diethyl ketone, methyl ethyl ketone and the like.

This hydrophilic organic solvent can be used alone or as a mixture of two or more solvents. In the present invention, i-propanol, methyl carbitol, methyl ethyl ketone, ethylene glycol alone or two or more thereof are used. Is preferably used.

The mixing ratio of the component (c) in the coating composition is based on 100 parts by weight of the inorganic filler (a).
It is 10 to 100 parts by weight, preferably 10 to 50 parts by weight. If the compounding ratio of the component (c) is less than 10 parts by weight, the viscosity of the coating composition will increase too much, and the storage stability will decrease,
The dispersing power of the components (a) and (b) deteriorates,
When the amount exceeds 100 parts by weight, the storage stability is improved, but other components are relatively reduced, and the adhesion of the obtained overcoat layer is weakened, or the target film may be formed too thin. It may not be possible.

In addition to the components (a) to (c), various surfactants, dispersants, curing regulators, organic dyes and pigments, and other additives may be added to the coating composition of the present invention, if necessary. Agents can be included.

In the above coating composition, the components (a) and (b) are mixed, and if the component (c) is insufficient, the component (c) is further blended as necessary to obtain a total solid content. The concentration is preferably 15-85% by weight, more preferably 30-75% by weight.
% By weight. At this time, the coating composition is
By dispersing and filtering with a high-speed stirrer, a roll mill, a ball mill, or another disperser, a uniform and stable dispersion can be obtained.

For applying the coating composition to the undercoat layer,
Coating means such as brush coating, spraying, dipping, roll coating, and printing can be used. Here, in order to apply the coating composition to the surface of the undercoat layer,
The composition is applied in an amount of from 10 to 300 g, preferably from 40 to 200 g, in terms of solid content, per m ² per time. this,
It can be carried out several times, but usually, the total applied amount is ²⁰ to 600 g, preferably 60 to 300 g, in terms of solid content per m ² . Further, this coating film has a dry film thickness of usually 10 to 400 μm, preferably 20 to 2 μm.
00 μm.

After the above coating composition is applied, it is cured at room temperature at 5 to 30 ° C. for 1 to 5 hours, or easily cured at a low temperature of 80 to 180 ° C. for 2 to 30 minutes. And a coating film can be obtained.

The coating film thus obtained is composed mainly of (a) an inorganic filler and (b) a binder. For this reason, it protects the undercoat layer, has hydrophilicity, air permeability, moderately ionizes the metal or rust preventive pigment of the undercoat layer, and plays a role of a protective film that does not form a local battery on the base material, and has durability. An excellent film is obtained.

[0029]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples unless it exceeds the scope of the claims. In the examples, parts and% are based on weight unless otherwise specified.

Experiment No. 1 to 16 (Example), Experiment N
o. 17 to 23 (Comparative Examples) Preparation of Coating Compositions A to G Seven types of compositions A to G shown in Table 1 were prepared . This composition is prepared by adding the components (a) to (b) and, if necessary, the component (c) and / or other additives to a stirring tank, stirring the mixture lightly, and then stirring at a high speed (15,000 rpm).
pm) for 30 minutes and filtered through 50-100 mesh.

The symbols in Table 1 represent the following. (A) Inorganic filler (a) -1; kaolin (average particle size = 4-5 μm) (a) -2; silica sand (average particle size = 20 μm) (a) -3; silicon dioxide (average particle size = 2) (A) -4; aluminum oxide (average particle size = 0.7 μm)
m) (a) -5: Titanium oxide white (average particle size = 0.5 μm) (a) -6: Iron oxide yellow (average particle size = 0.5 μm) (a) -7: Iron / manganese synthetic oxide Black (Average particle size =
(A) -8; nickel (average particle size = 2 μm) (a) -9; potassium titanate whisker (average length =
(A) -10; aluminum hydroxide (average particle size = 3 μm)
m) (a) -11; zeolite carrying silver (average particle size = 1.5 μm)
m)

(B) Binder (b) -1; amine silicate [NS Co., Ltd., NS
-20 (SiO ₂ concentration = about 20%] (b) -2; silicic acid amine (manufactured by Nissan Chemical Industries, Ltd.), lithium 25 (SiO ₂ concentration = about 25%] (b) -3; lithium silicate [ Nissan Chemical Industry Co., Ltd.
Cas 75 (SiO ₂ concentration = about 20%) (b) -4; colloidal silica [Kata Kasei Kogyo Co., Ltd.]
Cataloid SN (SiO ₂ concentration = about 20%) (b) -5; Ethylene-vinyl acetate copolymer emulsion [EV-15 (Non-volatile content = 55%) manufactured by Dainippon Ink and Chemicals, Inc.) b) -6: acrylic-styrene copolymer emulsion (acrylic emulsion) [Dainippon Ink & Chemicals, Inc., EC-905 (non-volatile content = 50%)]

(C) Water or hydrophilic organic solvent (c) -1; ion-exchanged water (c) -2; ethylene glycol (d) other components (d) -1; nonionic dispersant (d) -2 A silicon-based leveling agent (d) -3; aluminum orthophosphate

[0034]

[Table 1]

Preparation of rust preventive paint for undercoating In accordance with the formulation shown in Table 2, rust preventive paints for undercoat were prepared.

[0036]

[Table 2]

Preparation of Test Pieces for Evaluation Test On the surfaces of various steel plates shown in Tables 3 and 4, a metal or an undercoat layer is formed by coating, and the surface of the undercoat layer is coated with the paint prepared in Table 1. The composition was applied and cured, and test pieces No. 1 to 16 for evaluation test were applied.
(Example) and test pieces Nos. 17 to 23 (comparative examples) were prepared. The base steel sheet was subjected to an alkali treatment as a base treatment before applying the coating composition. An air spray gun was used for applying the coating composition, and an electric oven was used for heating. Tables 3 and 4 show the number of application times and curing conditions of the coating composition. The dimensions of the test pieces are all 150 mm × 70 mm.

[0038]

[Table 3]

[0039]

[Table 4]

Rust Prevention Test (Salt Water Spray Test )
A cross cut (an X-shaped cut is made on the diagonal line of the test piece) so that the iron skin is visible, and the test piece is subjected to a 5% saline solution according to JIS K-5400-9.1. Was sprayed continuously for a predetermined time, and the appearance of the coating film was observed. Table 5 shows the results.

[0041]

[Table 5]

In Table 5, も の indicates that no rust was found, △ indicates that rust was found on the end face or the cross cut part, and rust was found on the end face or the cross cut part. The product was evaluated as x. Note that-
Is not evaluated.

Physical property test The above test pieces Nos. 1 to 16 (Examples) were
After storage for days, the following physical property tests were performed. Table 6 shows the results. Adhesion: Based on JIS K-5400-8.5.1 cross cut test. Hardness: Based on JIS K-5400-8.4.2 pencil scratch test. Impact resistance: JIS K-5400-8.3.1 (25.
4 mm, 300 g × 20 cm). Warm water resistance: After the test pieces were stored in warm water at 60 ° C. for 14 days, the appearance of the coating film was observed. Weather resistance: Using a sunshine carbon arc type weather meter based on JIS K5400-9.8.1,
After the test piece was exposed for 3,000 hours, the appearance of the coating film was observed. In addition, evaluation judgment of the appearance observation result of a coating film was based on the following. ：: No peeling of the coating film was observed, and there was no change in appearance. X: Peeling, cracking or melting, stain, and discoloration are observed in part or all of the coating film. -: No evaluation Table 6 shows the results of the above evaluation test.

[0044]

[Table 6]

Salt Spray Test after Weathering Test Using test pieces Nos. 1-4 and Nos. 6-15 after the weathering test in Table 6, the same salt spraying for 1,200 hours as described above was performed. The test was performed. Table 7 shows the results. In addition, what did not generate | occur | produce rust was evaluated with (circle).

[0046]

[Table 7]

[0047]

The rust-preventive film of the present invention comprises: a corrosion-resistant metal layer covering the surface of iron or a coating film containing an anti-rust pigment (undercoat layer);
It is composed of a hydrophilic, air-permeable protective film (overcoat layer) for protecting it. Therefore, the rust prevention film of the present invention has the following effects. Since the protective film (overcoat layer) is hydrophilic, appropriate ionization of the coating film containing the coating metal or the rust-preventive pigment is possible, and the edge portion and the damaged portion are covered. Since the protective film (overcoat layer) has excellent drying properties, no dew condensation occurs at the interface. Since the protective film (overcoat layer) is a uniform hydrophilic and air-permeable film, there is no local potential difference, and a local battery cannot be formed. Unlike conventional environmental barrier coating processing, processing is simple and cost can be greatly reduced. The coating composition for forming the overcoat layer is water-based and easy to handle, and after application, at room temperature for 1 to 3 hours, or 80 to 1
It dries and hardens in about 2 to 30 minutes under heating at 80 ° C., and is excellent in workability.

Claims (4)

[Claims] At least one metal selected from the group consisting of zinc, tin, lead, aluminum, chromium, nickel and copper is coated on a surface of a substrate mainly composed of iron by a hot-dip plating method or an electroplating method. , At least one kind of rust-preventive pigment selected from the group consisting of zinc powder, zinc chromate, lead tin, basic lead chromate, strontium chromate and lead chromate, coated by a permeation plating method, a thermal spraying method or a chemical plating method. A rust-preventive film comprising an undercoat layer coated with a paint containing the composition, and an overcoat layer formed by applying a coating composition containing the following components (a) to (c) as main components and curing by drying at room temperature or by heating at low temperature. . (A) 100 parts by weight of inorganic filler (b) 5 to 30 parts by weight of binder in terms of solid content (c) 10 to 100 parts by weight of water and / or hydrophilic organic solvent 2. The coating composition for forming an overcoat layer, wherein (a) the inorganic filler has an average particle size or average length of 0.0
2-50 μm and water-insoluble, inorganic extender pigment,
The rust preventive film according to claim 1, which is at least one selected from the group consisting of a functional pigment, an inorganic pigment, and a metal powder. 3. The coating composition for forming an overcoat layer, wherein the binder (b) is at least one selected from the group consisting of silicate, colloidal silica, colloidal alumina and synthetic resin. Anti-rust film as described. 4. A hot-dip plating method, an electro-plating method, and a method according to claim 1, wherein
After coating by the infiltration plating method, the thermal spraying method or the chemical plating method, or applying the paint according to claim 1 to form an undercoat layer, applying the paint composition according to claim 1, drying at room temperature or A coating method for a rust-preventive film, wherein the coating is cured by drying at a low temperature to form an overcoat layer.