Method for forming multilayer coating film

A method for forming a multilayer coating film comprising the following steps in sequence:

• step (1):
• applying an aqueous primer paint (W) to a plastic molded article to form an uncured primer coating film;
• step (2):
• preheating the uncured primer coating film;
• step (3):
• applying an aqueous base paint (X) to the preheated primer coating film to form an uncured base coating film;
• step (4):
• applying an effect pigment dispersion (Y) to the uncured base coating film to form an uncured effect coating film;
• step (5):
• preheating the uncured base coating film and the uncured effect coating film;
• step (6):
• applying a clear paint (Z) to the preheated effect coating film to form an uncured clear coating film;
• and
• step (7):
• heating the preheated primer coating film, the preheated base coating film, the preheated effect coating film, and the uncured clear coating film at a temperature in the range of 110 to 130° C. to simultaneously cure the coating films, wherein
• the aqueous base paint (X) comprises
• a hydroxy-containing acrylic resin particle (X1) having a core-shell structure wherein the shell has an acid value of 90 to 100 mg KOH/g;
• a hydroxy-containing acrylic resin particle (X2) having a core-shell structure wherein the shell has an acid value of 110 to 120 mg KOH/g;
• and
• an associative thickener (X3);
• the ratio by mass of the hydroxy-containing acrylic resin particle (X1) to the hydroxy-containing acrylic resin particle (X2) contained in the aqueous base paint (X1/X2) is in the range of 70/30 to 30/70;
• the effect pigment dispersion (Y) contains
• a flake-aluminum pigment (Y1),
• a cellulose-based rheology control agent (Y2),
• a surface-adjusting agent (Y3), and
• water, and
• the effect pigment dispersion (Y) has a solids content in the range of 2 to 6 mass;
• and
• the effect coating film after curing has a film thickness of 0.5 to 1.5 μm.