JP4859099B2 - Painting method - Google Patents

Description

  The present invention relates to a coating method capable of forming a coating film excellent in stain resistance. More specifically, it can be used for surface finishing of various materials such as concrete, mortar, metal, glass, porcelain tiles, siding boards, extrusion boards, plastics, etc., and mainly contributes to the protection of structural bodies such as buildings and civil engineering structures. It is related to the painting method.

  In recent years, in the field of paints used for construction and civil engineering structures, a shift from solvent-based paints using organic solvents to water-based paints using water as a solvent is being attempted. This is performed for the purpose of reducing the health hazards of painters and residents, and for the purpose of reducing air pollution.

  Some known water-based paints have the same level of performance as solvent-based paints in terms of weather resistance, water resistance, and the like. However, with particular attention to contamination, even water-based paints that are said to be low-contamination are not as high as solvent-type low-contamination paints. In addition, a coating film made of a water-based paint generally has a coating film hardness lower than a coating film made of a solvent-type paint, and tends to have a high penetration when contaminated substances adhere. Therefore, once a contaminant has adhered, it is often difficult to remove the dirt from the coating surface.

  In order to deal with such problems, WO94 / 06870 (Patent Document 1) discloses a technique of blending organosilicate and / or a condensate thereof as a low-contamination agent in a paint. The technique described in Patent Document 1 includes an organosilicate and / or a condensate thereof in a paint, and acid-treats the paint film formed by the paint to make the paint film surface hydrophilic, thereby producing an oily contaminant. It is characterized in that the adhering contaminants are washed away together with water droplets such as rain.

However, in the case of Patent Document 1 in which an organosilicate and / or a condensate thereof are simply added, the hydrolysis condensation reaction that occurs when an organosilicate and / or a condensate thereof is added to an aqueous paint cannot be controlled. The viscosity of the paint suddenly increased in a short time, and the whole paint was gelled, so that it was not possible to perform normal painting work. Furthermore, even when coating is performed immediately after mixing and a coating film is formed, there is a disadvantage that the resistance to penetration of contaminants is poor.

  On the other hand, JP-A-11-343462 (Patent Document 2) discloses a coating material containing a modified organosilicate obtained by reacting a polyalkylene glycol compound with an organosilicate and / or a condensate thereof. . In the water-based paint described in Patent Document 2, the problem in Patent Document 1 is improved by modifying the organosilicate to enhance the compatibility with the water-based resin.

  However, after applying the water-based paint described in Patent Document 2 and drying the coating film, it is difficult to obtain sufficient adhesion even if the same paint is applied again on the coating film. The allowable range of the overcoating process may be limited. Further, in such a water-based paint, there is room for further improvement in order to exhibit a higher level of stable contamination resistance.

WO94 / 06870 JP-A-11-343462

  The present invention has been made in view of the above-mentioned problems, and in a coating method for obtaining a coating film having stain resistance by repeatedly applying a water-based paint, sufficiently ensuring mutual coating film adhesion, And it aims at improving the stain resistance of the formed coating film.

  As a result of intensive studies to achieve the above-mentioned object, the present inventor preferably uses a combination of a specific synthetic resin emulsion, a hydrazide compound, and an alkoxysilane-modified condensate as a coating material used for the overcoating step. As a result, the present invention has been completed.

  That is, the present invention relates to the following coating method.

1. In the coating method in which the same kind of water-based paint is repeatedly applied, the water-based paint includes an epoxy group-containing synthetic resin emulsion (A), a hydrazide compound (B) having a hydrazide group and a nitrogen-containing skeleton other than the hydrazide group, and polyoxyalkylene. A modified condensate (C) of an alkoxysilane having a group, and 0.1 to 20 parts by weight of the hydrazide compound (B) with respect to 100 parts by weight of the solid content of the synthetic resin emulsion (A). A coating method characterized by using a paint in which 1 to 40 parts by weight of a modified silane condensate (C) in terms of SiO ₂ is used. 2. In the coating method in which the same kind of water-based paint is repeatedly applied, the water-based paint includes an epoxy group-containing synthetic resin emulsion (A), a hydrazide compound (B) having a hydrazide group and a nitrogen-containing skeleton other than the hydrazide group, and polyoxyalkylene. A modified condensate (C) of an alkoxysilane having a group, and 0.1 to 20 parts by weight of the hydrazide compound (B) with respect to 100 parts by weight of the solid content of the synthetic resin emulsion (A). Using a coating in which 1 to 40 parts by weight of the modified silane condensate (C) is converted into SiO ₂ , the first coating of the aqueous coating is performed to form a lower coating, After curing and drying, a second coating of the water-based paint is performed to form an upper layer coating film.

  According to the present invention, in the coating method for obtaining a coating film having stain resistance by recoating with a water-based paint, mutual coating film adhesion can be sufficiently ensured, and further, the formed coating film resistance is improved. Contamination can be dramatically increased.

  Hereinafter, the present invention will be described in detail based on the embodiments.

  The water-based paint used in the present invention includes a synthetic resin emulsion (A) having a reactive functional group capable of reacting with a hydrazide group (hereinafter also referred to as “component (A)”), a hydrazide group and a nitrogen-containing skeleton other than the hydrazide group. As an essential component, a hydrazide compound (B) having a silane (hereinafter also referred to as “component (B)”) and a modified condensation product (C) of an alkoxysilane having a polyoxyalkylene group (hereinafter also referred to as “component (C)”) Is included.

  Among these, the component (A) is various synthetic resin emulsions having a reactive functional group capable of reacting with a hydrazide group, and the resin skeleton is not particularly limited as long as it has such a functional group. For example, an acrylic resin emulsion, an acrylic resin Silicon resin emulsion, fluororesin emulsion, urethane resin emulsion and the like can be used.

  Specifically, as the component (A), a synthetic resin emulsion obtained by copolymerization of a monomer having a reactive functional group capable of reacting with a hydrazide group and a monomer copolymerizable therewith can be used. Examples of the monomer having a reactive functional group capable of reacting with a hydrazide group include carbonyl group-containing monomers such as acrolein, diacetone (meth) acrylamide, vinyl methyl ketone, vinyl ethyl ketone, and vinyl butyl ketone; glycidyl (meth) acrylate And epoxy group-containing monomers such as diglycidyl (meth) acrylate. The weight ratio of such monomers to the total monomer weight is usually 0.1 to 40% by weight, preferably 0.5 to 20% by weight. In the present invention, an epoxy group-containing synthetic resin emulsion containing an epoxy group-containing monomer as a monomer component is particularly suitable.

  Examples of the monomer component copolymerizable with the above monomer include methyl (meth) acrylate, ethyl (meth) acrylate, isopropyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth) acrylate, 2-ethylhexyl ( (Meth) acrylic acid esters such as meth) acrylate and cyclohexyl (meth) acrylate; carboxyl group-containing monomers such as acrylic acid, methacrylic acid, crotonic acid, maleic acid or monoalkyl esters thereof; N-methylaminoethyl (meth) acrylate Amino group-containing monomers such as dimethylaminoethyl (meth) acrylate, dimethylaminoethyl vinyl ether and N- (2-dimethylaminoethyl) acrylamide; pyridine monomers such as vinylpyridine; 2-hydride Hydroxyl-containing monomers such as xylethyl (meth) acrylate and hydroxypropyl (meth) acrylate; vinyl ester monomers such as vinyl acetate and vinyl propionate; nitrile group-containing monomers such as acrylonitrile and methacrylonitrile; styrene, 2-methylstyrene, vinyl Aromatic monomers such as toluene; Amide group-containing monomers such as acrylamide, methacrylamide and diacetone acrylamide; Vinyltrimethoxysilane, vinyltriethoxysilane, γ- (meth) acryloyloxypropyltrimethoxysilane, γ- (meth) acryloyl Examples thereof include alkoxysilyl group-containing monomers such as oxypropyltriethoxysilane; vinylidene halide monomers such as vinylidene chloride and vinylidene fluoride. These can be used alone or in combination of two or more.

  The glass transition temperature (hereinafter referred to as “Tg”) of the component (A) can be appropriately set, but is usually −10 to 80 ° C., preferably 0 to 60 ° C. When Tg is lower than −10 ° C., it is difficult to obtain a sufficient effect in stain resistance, and when it is higher than 80 ° C., cracks and the like are likely to occur in the coating film.

  When the component (A) contains a carboxyl group-containing monomer, the weight ratio of the monomer to the total monomer weight is 5% or less, more preferably 2% or less, and further 1% or less. By limiting to such a range, contamination resistance can be further enhanced.

  Although the manufacturing method of (A) component is not specifically limited, For example, it can manufacture by methods, such as batch polymerization, monomer dripping polymerization, emulsion monomer dripping polymerization, as an emulsion polymerization method.

  As the emulsifier used for the polymerization, it is necessary to use an emulsifier that can ensure the stirring stability and low-temperature stability of the component (A). The type is appropriately selected from anionic, cationic, nonionic, amphoteric, nonionic-cationic, nonionic-anionic, non-reactive, and reactive, and used alone or in combination. To do. Among these, the use of a reactive emulsifier having an unsaturated double bond is preferable because it can improve water resistance. Furthermore, the combined use of a nonionic reactive emulsifier and an anionic reactive emulsifier is preferable because it can improve water resistance, stirring stability and low temperature stability, and can also improve stain resistance.

  In the present invention, a hydrazide compound having a hydrazide group and a nitrogen-containing skeleton other than the hydrazide group is used as the component (B). This (B) component is an essential component for ensuring the coating film adhesion at the time of overcoating. The reason why such an effect is obtained is not clear, but the following action mechanism is generally estimated. That is, in this invention, the compatibility of (B) component with respect to (C) component is high, and (B) component with the (C) component becomes easy to orientate on the coating-film surface in a coating-film formation process. As a result, a hydrazide group is present on the surface of the first layer coating film, and when the second layer is overcoated, the first layer hydrazide group and the second layer emulsion functional group. It is considered that the effect of improving the adhesion is expressed by the action. In addition to such an effect of improving adhesion, in the present invention, a remarkable effect of improving stain resistance can be obtained by blending component (B). Such an effect is presumed to be because the component (B) acts as a catalyst for the component (C) and contributes to the reaction of the component (C).

  The nitrogen-containing skeleton in the component (B) may be a chain, branched or cyclic organic group having a nitrogen atom in the skeleton, and examples thereof include a hydantoin skeleton and a piperazine skeleton. In particular, the hydantoin skeleton is preferred in the present invention. Examples of the hydrazide compound having a hydrazide group and a hydrantoin skeleton include 1,3-bis (hydrazinocarboethyl) -5-isopropylhydantoin and 1,3-bis (hydrazinocarboethyl) -5- (2-methylmercapto And ethyl) hydantoin and 1-hydrazinocarboethyl-3-hydrazinocarboisopropyl-5- (2-methylmercaptoethyl) hydantoin. In particular, 1,3-bis (hydrazinocarboethyl) -5-isopropylhydantoin is preferred in the present invention.

In general, dihydrazide compounds of saturated aliphatic carboxylic acids such as oxalic acid dihydrazide, malonic acid dihydrazide, succinic acid dihydrazide, glutaric acid dihydrazide, adipic acid dihydrazide, and sebacic acid dihydrazide are widely used as hydrazide compounds in the paint field. Yes. Such a hydrazide compound has a structure having a hydrazide group at both ends of a hydrocarbon group represented by — (CH ₂ ) _n —, and does not have a nitrogen-containing skeleton other than the hydrazide group. Even if it uses, the remarkable effect like this invention cannot be acquired.

  The component (B) is added in the range of 0.1 to 20 parts by weight (preferably 0.5 to 10 parts by weight) with respect to 100 parts by weight of the solid content of the component (A). When the mixing ratio of the component (B) is within such a range, the effects of the present invention can be sufficiently obtained. When the amount of the component (B) is less than 0.1 parts by weight, it is difficult to obtain an effect of improving adhesion and an effect of improving stain resistance. On the other hand, when the component (B) is more than 20 parts by weight, the water resistance and stain resistance of the formed coating film may be lowered, and it is difficult to obtain a sufficient gloss in the gloss enamel.

The modified condensation product (C) of alkoxysilane having a polyoxyalkylene group in the present invention serves as a low-contamination agent and is an essential component for imparting stain resistance to the water-based paint of the present invention. .
As the component (C), those having 1 to 4 carbon atoms in the repeating unit of the polyoxyalkylene group are preferred, and polyoxyethylene groups having 2 carbon atoms are more preferred. Moreover, it is preferable that the average molecular weights of a polyoxyethylene group are 150-2000.
(C) As an alkoxyl group of a component, a C1-C4 thing is preferable and a C2-ethoxy group is more preferable.

Component (C) can be obtained by a known method. Specific production methods include, for example, a method of transesterifying one or more of the alkoxysilane condensates with one or more polyoxyalkylene group-containing products, etc. Is mentioned. That is, the general formula (R ¹ O) _4-a -SiR ² _a
(Wherein R ¹ represents an alkyl group having 1 to 4 carbon atoms, R ² represents an alkyl group, an aryl group, an aralkyl group, or an alkoxyl group having 1 to 4 carbon atoms, and a represents an integer of 0 to 2) ) Represented by a general formula R ³ — (OC _n H _2n ) _m — (hereinafter referred to as “component p”). R ⁴
(In the formula, R ³ represents a hydrogen atom, an alkyl group, an epoxy group, or an acyl group, R ⁴ represents a hydroxyl group, an alkyl group, an alkoxyl group, an epoxy group, an acyl group, or a carboxyl group, and n is an integer of 1 to 4. , M represents an integer of 1 to 20) and is produced by transesterification with a polyoxyalkylene group-containing compound (hereinafter referred to as “q component”). Particularly, in the transesterification reaction, since it is necessary to have at least one hydroxyl group at the terminal, a q component having a hydroxyl group at the terminal is used. When performing the transesterification reaction, a transesterification catalyst can be added as necessary.

  Specific examples of the p component monomer include tetramethoxysilane, tetraethoxysilane, tetra-n-propoxysilane, tetra-i-propoxysilane, tetra-n-butoxysilane, tetra-i-butoxysilane, and tetra-t. -Butoxysilane, methyltrimethoxysilane, methyltriethoxysilane, methyltripropoxysilane, methyltributoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, ethyltripropoxysilane, ethyltributoxysilane, propyltrimethoxysilane, propyl Triethoxysilane, propyltripropoxysilane, propyltributoxysilane, butyltrimethoxylane, butyltriethoxysilane, butyltripropoxysilane, butyltributoxysilane, dimethyl Silane, dimethyl diethoxy silane, dimethyl dipropoxy silane, dimethyl dibutoxy silane, diethyl dimethoxy silane, diethyl diethoxy silane, diethyl di-propoxysilane, and condensate of diethyl dibutoxy silane. These can be used individually or in mixture of several types.

  Carbon number of the alkoxysilyl group of the p component is usually 1 to 4 (preferably 2). Further, the average condensation degree of the p component is preferably 1-20. When the average degree of condensation exceeds 20, it is not preferable because handling becomes inconvenient.

  Specific examples of the q component include polyoxyethylene glycol, polyoxyethylene glycol monoalkyl ether, polyoxyethylene-propylene glycol, polyoxyethylene-tetramethylene glycol, polyoxyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, poly Oxyethylene diglycolic acid, polyoxyethylene glycol vinyl ether, polyoxyethylene glycol allyl ether, polyoxyethylene glycol diallyl ether, polyoxyethylene alkyl ether, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene fatty acid ester, polyoxyethylene alkylamine Etc. These compounds can be selected from one or a combination of two or more.

  The average molecular weight of the q component is preferably 150 to 2000. When the average molecular weight is less than 150, the stability when the low-contaminating agent obtained by the transesterification reaction with the p component is mixed with the paint tends to be inferior. With gloss paint, excellent gloss is obtained. It will disappear. Conversely, if the average molecular weight of the q component exceeds 2000, the water resistance and hardness of the cured coating film tend to decrease.

(C) amount of component, 100 parts by weight of the solid content of the component (A), in the range of 1 to 40 parts by weight in terms of SiO _2. When the component (C) is less than 1 part by weight, the effect of reducing contamination cannot be expressed. When the component (C) exceeds 40 parts by weight, the viscosity stability of the water-based paint tends to decrease, and gloss paint may cause a decrease in gloss.

The SiO ₂ conversion in the present invention means the weight remaining as silica (SiO ₂ ) when a compound having a Si—O bond such as alkoxysilane or silicate is completely hydrolyzed and then baked at 900 ° C. Expressed in minutes.
In general, alkoxysilanes and silicates have a property of reacting with water to cause a hydrolysis reaction to form silanol, and further causing a condensation reaction between silanols or between silanol and alkoxy. When this reaction is performed to the ultimate, silica (SiO ₂ ) is obtained. These reactions are RO (Si (OR) ₂ O) _n R + (n + 1) H ₂ O → nSiO ₂ + (2n + 2) ROH
(R represents an alkyl group. N is an integer.)
It is expressed by the reaction formula. The SiO ₂ conversion in the present invention is the conversion of the amount of the remaining silica component based on this reaction formula.

  In the present invention, since the component (C) reacts with water, it is better to avoid contact with the component (A) containing water for a long period of time. Accordingly, it is desirable that the water-based paint in the present invention be handled as a multi-component type paint having two or more components at the distribution stage and mixed at the time of use. As a form in this case, for example, (A) component as the first component, (B) component as the second component, (C) component mixture, or (A) component and (B) component mixture as the first component The second component can be a mixture of component (C).

In addition to the above-mentioned components, the water-based paint in the present invention can be blended with organic solvents, inorganic color pigments, organic color pigments, extender pigments, aggregates, etc. that are usually used in paints, if necessary. Plasticizers that do not affect the invention, film-forming aids, antiseptics, antifungal agents, antifoaming agents, thickeners, leveling agents, pigment dispersants, coupling agents, anti-settling agents, anti-sagging agents, matte Additives such as an agent, an ultraviolet absorber, an antioxidant, and a light stabilizer can be used alone or in combination.
In addition, a curing catalyst other than the component (B) can be added. Examples of such a curing catalyst include organic tin compounds such as dibutyltin dilaurate, dibutyltin malate, dioctyltin dilaurate, and dioctyltin malate; phosphoric acid such as phosphoric acid, monomethyl phosphate, monoethyl phosphate, monooctyl phosphate; Phosphate esters; Adducts of epoxy compounds such as propylene oxide, butylene oxide, glycidyl methacrylate, γ-glycidoxypropyltriethoxysilane, and Epicoat 828 with phosphoric acid and / or acidic monophosphate esters; organic titanate compounds; organic Aluminum compounds; organic zirconium compounds; acidic compounds such as maleic acid, adipic acid, azelaic acid, acid anhydrides thereof, paratoluenesulfonic acid; hexylamine, N.I. Examples thereof include amines such as N-dimethyldodecylamine; alkaline compounds such as sodium hydroxide. The addition amount of such a curing catalyst is about 0.1-20 weight part normally with respect to 100 weight part of solid content of (A) component.

The present invention is mainly used for protection of buildings, civil engineering structures, etc., for example, for surface finishing of various substrates such as concrete, mortar, metal, glass, porcelain tiles, siding boards, extrusion boards, plastics, etc. Can be used. These base materials may have an existing coating film. Prior to the application of the water-based paint, a surface treatment with a sealer, a surfacer, a filler or the like can be appropriately performed.
The water-based paint in the present invention is used as the final finishing material of such a substrate and is finished by two or more overcoats. Usually, the first coating is performed to form a lower layer coating, and after the coating is dried, the second coating is performed to form an upper coating. Here, the dried state of the lower layer coating film may be any of touch drying, semi-cured drying, and cured drying. In the present invention, however, a remarkable effect is obtained particularly when the lower layer coating film is in a cured and dried state. Can be obtained. Specifically, in a standard state (temperature 23 ° C./relative humidity 50%), a remarkable effect can be obtained when a drying time of 3 days or more, further 7 days or more is provided.
As a method for applying the water-based paint, various methods such as brush coating, spray coating, and roller coating can be appropriately employed. Dry building materials can be pre-coated at factories and the like.

  Examples and Comparative Examples are shown below to clarify the features of the present invention.

[Synthesis of alkoxysilane-modified condensate]
100 parts by weight of an ethyl silicate condensate having an average molecular weight of 750 (average molecular weight 750, residual silica ratio of 40% by weight), which is a condensate of tetraethoxysilane, and polyoxyethylene glycol # 200 having an average molecular weight of 200 (manufactured by Wako Pure Chemical Industries, Ltd.) ) 106.7 parts by weight, and 0.02 part by weight of dibutyltin dilaurate was added as a catalyst, followed by deethanol reaction at 75 ° C. for 8 hours to synthesize an alkoxysilane-modified condensate. The residual silica ratio of the alkoxysilane-modified condensate was 20.9% by weight.

[Preparation of paint]
In accordance with the formulation shown in Table 1, the following raw materials were mixed by a conventional method, and then the alkoxysilane-modified condensate was mixed to produce an aqueous coating material.
Synthetic resin emulsion 1 (methyl methacrylate, 2-ethylhexyl acrylate, styrene, glycidyl methacrylate, methacrylic acid copolymer, solid content 50% by weight, pH 6.2, Tg 14 ° C.)
Synthetic resin emulsion 2 (methyl methacrylate, 2-ethylhexyl acrylate, styrene, methacrylic acid copolymer, solid content 50% by weight, pH 6.1, Tg 14 ° C.)
-Color pigment: Titanium oxide-Film-forming aid: 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate-Catalyst: Dibutyltin dilaurate-Thickener: Urethane thickener-Antifoaming agent: Silicone-based antifoaming agent / hydrazide compound 1: 1,3-bis (hydrazinocarboethyl) -5-isopropylhydantoin / hydrazide compound 2: adipic acid dihydrazide

[Test method]
The following tests were conducted on the water-based paint obtained by the above method.

-Specular Gloss After applying a water-based paint to a transparent glass plate of 150 x 120 x 3 mm so that the WET film thickness is 150 μm and drying and curing in standard conditions (temperature 23 ° C, relative humidity 50%) for 2 days The gloss value at an angle of 60 degrees was measured.

・ Water resistance Spray coating of SK # 1000 primer (epoxy resin primer; manufactured by SK Kaken Co., Ltd.) on a flexible plate of 150 × 70 × 3 mm to a dry film thickness of 30 μm, followed by drying for 8 hours under standard conditions went.
Next, the prepared coating composition was spray-coated so that the dry film thickness was 40 μm to prepare a test body. The prepared specimen was dried and cured for 7 days in a standard state, and the gloss retention after being immersed in warm water at 50 ° C. for 1 hour was calculated. The evaluation criteria were a gloss retention ratio of 95% or more as “」 ”, 90% or more and less than 95% as“ ◯ ”, 80% or more and less than 90% as“ Δ ”, and less than 80% as“ X ”.

Contamination resistance A 150 × 75 × 0.8 mm aluminum plate was spray-coated with a SK # 1000 primer so as to have a dry film thickness of 30 μm, and dried in a standard state for 8 hours.
Next, the prepared coating composition was spray-coated so that the dry film thickness was 40 μm to prepare a test body. The prepared specimen was dried and cured in a standard state for 7 days, and then a 15 wt% carbon black aqueous dispersion paste solution was dropped onto the coating surface so as to have a diameter of 20 mm and a height of 5 mm in a constant temperature room at 50 ° C. For 2 hours. Thereafter, the film was washed in running water, and the degree of contamination on the coating film surface was visually evaluated in four stages (excellent: ◎>◯>Δ> ×: inferior).

-Adhesion at the time of recoating The SK # 1000 primer was spray-coated on a 150 × 75 × 0.8 mm aluminum plate so as to have a dry film thickness of 30 μm, and dried for 8 hours in a standard state.
Next, after spray coating the prepared coating composition so that the dry film thickness becomes 40 μm and curing under each condition (standard state 2 days, standard state 10 days, 50 ° C. 2 days, 50 ° C. 10 days) Further, the same coating composition was spray-coated so as to have a dry film thickness of 40 μm and cured for 3 days in a standard state to obtain a test body. The prepared specimens were evaluated for adhesion by a cross-cut tape method according to JIS K 5600-5-6. Evaluation criteria were “◯” when the defect area was 5% or less, “Δ” when the defect area was 5 to 35%, and “X” when the defect area was 35% or more.

[Test results]
The test results are shown in Table 2. In Examples 1 to 4, good results could be obtained in any test.

Claims (2)

In the coating method in which the same kind of water-based paint is repeatedly applied, the water-based paint includes an epoxy group-containing synthetic resin emulsion (A), a hydrazide compound (B) having a hydrazide group and a nitrogen-containing skeleton other than the hydrazide group, and polyoxyalkylene. A modified condensate (C) of an alkoxysilane having a group, and 0.1 to 20 parts by weight of the hydrazide compound (B) with respect to 100 parts by weight of the solid content of the synthetic resin emulsion (A). A coating method characterized by using a paint in which 1 to 40 parts by weight of a modified silane condensate (C) in terms of SiO ₂ is used. In the coating method in which the same kind of water-based paint is repeatedly applied, the water-based paint includes an epoxy group-containing synthetic resin emulsion (A), a hydrazide compound (B) having a hydrazide group and a nitrogen-containing skeleton other than the hydrazide group, and polyoxyalkylene. A modified condensate (C) of an alkoxysilane having a group, and 0.1 to 20 parts by weight of the hydrazide compound (B) with respect to 100 parts by weight of the solid content of the synthetic resin emulsion (A). Silane modified condensate (C) is SiO ₂ Using the paint blended in an amount of 1 to 40 parts by weight, the first coating of the water-based paint is performed to form a lower layer coating, and after the lower layer coating is cured and dried, the second coating of the water-based coating is performed. A coating method characterized in that an upper layer coating film is formed by performing a second coating.