Method for forming multilayer coating film

A method for forming a multilayer coating film by sequentially carrying out, in order:

• (1) a step of applying a primer paint (P) on a substrate that comprises a metal material and a plastic material so as to form an uncured primer coating film;
• (2) a step of applying an aqueous first colored paint (X) on the uncured primer coating film obtained in step (1) so as to form an uncured first colored coating film;
• (3) a step of applying an aqueous second colored paint (Y) on the uncured first colored coating film obtained in step (2) so as to form an uncured second colored coating film;
• (4) a step of applying a clear paint (Z) on the uncured second colored coating film obtained in step (3) so as to form an uncured clear coating film;
• and
• (5) a step of heating the uncured primer coating film, the uncured first colored coating film, the uncured second colored coating film and the uncured clear coating film formed in steps (1) to (4) so as to simultaneously cure these 4 coating films,
• wherein the aqueous first colored paint (X) is a 1-package type paint composition containing a hydroxyl group-containing resin (X1) and a blocked polyisocyanate compound (X2), and the aqueous second colored paint (Y) is a 1-package type paint composition containing a hydroxyl group-containing resin (Y1) and a blocked polyisocyanate compound (Y2),
• wherein the clear paint (Z) is a 2-package type paint composition which contains a hydroxyl group-containing resin (Z1) having a hydroxyl value of 100 to 150 mg KOH/g, an aliphatic polyisocyanate compound and/or a derivative thereof (Z2), and a non-aqueous polymer dispersion liquid (Z3) having an hydroxyl value of substantially 0 mg KOH/g, and in which the solid content proportion in the non-aqueous polymer dispersion liquid (Z3) falls within a range of 20 to 30 mass % of the total quantity of solid content of the hydroxyl group-containing resin (Z1) and the aliphatic polyisocyanate compound and/or a derivative thereof (Z2), and a ratio of isocyanate groups to hydroxyl groups (NCO/OH) falls within a range of 0.9 to 1.2, and
• wherein the metal material comprises an electrodeposition paint on a surface thereof.