Method for forming a multilayer coating film

A method for forming a multilayer coating film, comprising:

• a step (1) of applying an aqueous 2-package first colored paint on both a metal member and a plastic member of an automobile outer panel so as to form an uncured first colored coating film;
• a step (2) of applying an aqueous 1-package second colored paint on the uncured first colored coating film obtained in step (1) so as to form an uncured second colored coating film;
• a step (3) of applying a solvent-based 2-package clear paint on the uncured second colored coating film obtained in step (2) so as to form an uncured clear coating film;
• and
• a step (4) of heating the uncured first colored coating film, the uncured second colored coating film and the uncured clear coating film formed in the steps (1) to (3) to 75 to 100° C. so as to simultaneously cure the coating films formed in the steps (1) to (3),
• wherein the aqueous 2-package first colored paint contains a hydroxyl group-containing polyester resin, a hydroxyl group-containing acrylic resin, a hydroxyl group-containing polyurethane resin, and a polyisocyanate compound as film-forming components, and the polyisocyanate compound contains an anionic hydrophilized polyisocyanate compound,
• wherein the solvent-based 2-package clear paint contains a hydroxyl group-containing acrylic resin and a polyisocyanate compound in a ratio of 1.5 to 2.0 equivalents of isocyanate groups in the polyisocyanate compound relative to 1 equivalent of hydroxyl groups in the hydroxyl group-containing acrylic resin, and
• wherein the polyisocyanate compound contains a polyisocyanate compound having a uretdione structure and a compound of a trimer or more of diisocyanate in a weight ratio of solid content within a range of 10/90 to 50/50.