A composition for curable paints, comprising(A) 100 parts by weight of a resin for curable paints, and(B) 0.5 to 10 parts by weight of chlorinated and graft-modified polypropylene which is produced by chlorinating polypropylene graft-modified with a grafting component selected from the group consisting of alpha,beta-unsaturated dicarboxylic acids, anhydrides thereof and esters thereof, and which has a chlorine content of 5 to 50% by weight, said graft-modified polypropylene having a degree of saponification of 5 to 150 mg/g and an intrinsic viscosity [.eta.] of 0.1 to 5 dl/g. A curable paint comprising the composition and a diluent.