Aqueous metallic coating composition and method for forming a multilayer coating film

An aqueous metallic coating composition comprising:

• (A) an aqueous film-forming resin which is at least one member selected from the group consisting of a hydroxy-containing acrylic resin (A1) and a hydroxy-containing polyester resin (A2), and the hydroxy-containing acrylic resin (A1) is a core-shell-type water-dispersible acrylic resin (A1′) comprising:
• a core copolymer (I) consisting of 0.1 to 30 mass % of a polymerizable unsaturated monomer having two or more polymerizable unsaturated groups per molecule and 70 to 99.9 mass % of a polymerizable unsaturated monomer having one polymerizable unsaturated group per molecule as monomer components;
• and
• a shell copolymer (II) consisting of 5 to 50 mass % of a hydrophobic polymerizable unsaturated monomer, 1 to 40 mass % of a hydroxy-containing polymerizable unsaturated monomer, and 10 to 94 mass % of other polymerizable unsaturated monomer(s) as monomer components;
• and
• the solids mass ratio of copolymer (I) to copolymer (II) is in the range of 10/90 to 90/10;
• (B) a curing agent;
• (C) a phosphoric acid group-containing resin having an acid value of 10 to 200 mg KOH/g and obtained by copolymerizing a phosphoric acid group-containing polymerizable unsaturated monomer (a) with other polymerizable unsaturated monomer(s) (b);
• the monomer (a) being represented by the formula
• wherein R1 represents a hydrogen atom or a methyl group, X represents —O— or —NH—, and R2 represents a C1-30 divalent organic group;
• (D) a nitrogen-containing compound represented by the formula
• wherein R3 represents a C4-22 monovalent organic group, R4 represents a C4-22 hydrocarbon group, a C4-22 acyl group or
• wherein R6 represents a C2-10 alkylene group, and n is an integer of 0 to 100, and when n is 2 or more, the R6's may be the same or different;
• and also wherein R5 represents a C2-10 alkylene group, m is an integer of 1 to 100, and when m is 2 or more, the R5's may be the same or different;
• and
• (E) a metallic pigment.