Method for forming multilayer coating film

A method for forming a multilayer coating film in which the following steps (1) to (4) are carried out in order on an article to be coated:

• step (1):
• applying a first coloring coating material (X) onto an article to be coated to form a first coloring coating film,
• step (2):
• applying a second coloring coating material (Y) onto the first coloring coating film formed in step (1), to form a second coloring coating film,
• step (3):
• applying a clear coating material (Z) onto the second coloring coating film formed in step (2) to form a clear coating film, and
• step (4):
• heat curing the first coloring coating film, the second coloring coating film and the clear coating film formed in steps (1) to (3) all at once,
• wherein:
• the first coloring coating material (X) contains a hydroxyl group-containing resin (x1), the second coloring coating material (Y) contains a hydroxyl group-containing resin (y1), the clear coating material (Z) contains a hydroxyl group-containing acrylic resin (a) and a polyisocyanate compound (b), and the polyisocyanate compound (b) contains an aliphatic triisocyanate compound (b1) with a molecular weight in the range of 200 to 350, and a polyisocyanate compound (b2) with a molecular weight in the range of 500 to 2000, wherein the content ratio of the aliphatic triisocyanate compound (b1) and the polyisocyanate compound (b2) is (b1)/(b2)=5/95 to 99/1, as the mass ratio of the aliphatic triisocyanate compound (b1)/polyisocyanate compound (b2),
• the aliphatic triisocyanate compound (b1) comprises an aliphatic triisocyanate compound selected from the group consisting of 1,8-diisocyanato-4-isocyanatomethyloctane, 2-isocyanatoethyl (2S)-2,6-diisocyanatohexanoate (common name:
• lysine triisocyanate), 2-isocyanatoethyl 2,6-diisocyanatohexanoate, 1,6,11-triisocyanatoundecane, and the combinations thereof;
• and
• the polyisocyanate compound (b2) comprises an aliphatic diisocyanate compound or a derivative thereof.