Composition comprising a polyester resin (A) made waterborne by adding 7-20 mass %, based on resin solids content of (A), of (a) a C1-3 alkoxy group-containing glycol monoalkyl ether to a polyester resin of resin acid value 25-35 mgKOH/g and neutralizing with a secondary amine and/or tertiary amine; a melamine resin (B) comprising methylated melamine resin and methyl/butyl mixed alkylated melamine resin, the content ratio based on resin solids content by mass of the methylated melamine resin to the methyl/butyl mixed alkylated melamine resin from 50/50 to 90/10, and the content ratio based on resin solids content by mass of component (A) to component (B) is 70/30 to 90/10, and a polypropylene glycol (C) comprising a number average molecular weight between 400 and 1,200, and in a mass percentage between 1 and 10 mass % in terms of the combined resin solids contents by mass of component (A) and component (B).