Patent ID: 8592000

Claim:
A method for forming a multilayer coating film on a coating object, which comprises successively carrying out the following steps (1)-(6): (1) a step of applying a water-based intermediate paint (X), which comprises, as a thermosetting resin component, a base resin (A) which is a combination of both a hydroxy-containing acrylic resin (A1) and a hydroxy-containing polyester resin (A2), and a crosslinking agent (B) which is a combination of both an amino resin (B1) and a blocked polyisocyanate compound (B2), to form an intermediate coating film, (2) a step of adjusting a solid content of the intermediate coating film which is formed in the step (1) to 70-100 mass %, (3) a step of forming a base coating film by applying onto the intermediate coating film as obtained in the step (2) a water-based base paint (Y), which comprises, as a thermosetting resin component, a base resin (A) which is a combination of both a hydroxy-containing acrylic resin (A1) and a hydroxy-containing polyester resin (A2), and a crosslinking agent (B) of an amino resin (B1), and which comprises 30-55 mass parts, per 100 mass parts of a solid resin content of the paint (Y), of an alcohol solvent having a boiling point of 170-250° C., (4) a step of adjusting a solid content of the base coating film which is formed in the step (3) to 70-100 mass % and pre-heating the intermediate coating film and the base coating film at 30-100° C. for 30 seconds-15 minutes, (5) a step of forming a clear coating film by applying onto the base coating film as obtained in the step (4) a clear paint (Z) comprising 30-70 mass parts of a carboxy-containing compound and 70-30 mass parts of a polyepoxide, per 100 mass parts of a solid resin content of the paint (Z), wherein the maximum storage modulus (G′1) of the paint (Z), as measured while being kept at 110° C. for 10 minutes at a stress of 0.6 Pa and frequency of 0.1 Hz, falls within a range of 0.001-30 Pa, and (6) a step of curing the intermediate coating film, base coating film and clear coating film, which are formed in the steps (1)-(5), simultaneously, by heating them at 100-120° C. for 3-10 minutes and thereafter further heating them at 130-160° C. for 10-30 minutes.