Method for forming multilayer film and multilayer film

A method for forming a multilayer film, comprising:

• a step (1) of applying an aqueous two-pack first colored coating material on both a metal member and a plastic member of an automobile outer panel to form an uncured first colored coating film;
• a step (2) of applying an aqueous one-pack white color coating material on the uncured first colored coating film obtained in the step (1) to form an uncured white coating film;
• a step (3) of setting the uncured white coating film obtained in the step (2) such that a resultant coating film has a solid content of 50% by mass or more;
• a step (4) of applying an aqueous one-pack interference color coating material on the uncured white coating film having the solid content of 50% by mass or more obtained in the step (3) to form an uncured interference color coating film;
• a step (5) of applying a solvent-based two-pack clear coating material on the uncured interference color coating film obtained in the step (4) to form an uncured clear coating film;
• and
• a step (6) of heating the uncured first colored coating film, the uncured white coating film, the uncured interference color coating film, and the uncured clear coating film formed in the steps (1) to (5) at a temperature of 75° C. or more and 100° C. or less to simultaneously cure these coating films to form a multilayer film,
• wherein the aqueous one-pack interference color coating material contains a vehicle-forming resin, and a titanium oxide-coated alumina flake pigment having a mean particle diameter in the range from 10 μm or more to 14 μm or less;
• the aqueous one-pack interference color coating material contains 15 parts by mass or more and 21 parts by mass or less of the titanium oxide-coated alumina flake pigment based on 100 parts by mass of the vehicle-forming resin;
• and the aqueous one-pack interference color coating material has a coating material solid content of 18% by mass or more and 22% by mass or less,
• wherein the interference color coating film has a cured film thickness of 4 μm or more and 7 μm or less,
• wherein the solvent-based two-pack clear coating material contains a hydroxyl group-containing acrylic resin and a polyisocyanate compound in a ratio such that isocyanate groups in the polyisocyanate compound is 1.5 equivalents or more and 2.0 equivalents or less relative to 1 equivalent of hydroxyl groups in the hydroxyl group-containing acrylic resin, and the polyisocyanate compound contains a polyisocyanate compound having a uretdione structure and a diisocyanate trimer or higher compound in a solid content mass ratio of 10/90 or more and 50/50 or less, and
• wherein the multilayer film has a lightness L* (15°) of 113 or more and 120 or less, a sparkle intensity Si (45°) of 3 or more and 5 or less, and a reciprocal of graininess 1/G of 0.4 or more and 0.5 or less, when measured by a multi-angle colorimeter.