A heat-curable, water-dispersible resin composition comprising 50 to 90% by weight of a film-forming polymer (A) obtained by copolymerizing an ethylenic monomer having a saturated C.sub.6 to C.sub.18 hydrocarbon group, a hydroxy-containing ethylenic monomer, an acidic group-containing ethylenic monomer and another ethylenic monomer, and 50 to 10% by weight of a hydroxy-terminated polyester resin (B), said resin (B) being grafted on said polymer (A) by transesterification, and at least part of the acidic groups in said polymer (A) being neutralized with a base (C); a method of producing said resin composition; a water-based paint composition comprising said resin composition and a curing agent; and a two-coat one-bake coating method which uses said paint composition as a base coat. This invention provides a water-based paint composition which shows good workability in a broad humidity condition range, and excellent in storage stability, paint film appearance and performance characteristics.