Abstract: The present disclosure keeps a topcoat film 3 from having a dent 6 in a portion of its surface corresponding to a pinhole 4 of a resin molded product 1. A method of the present disclosure includes: applying an undercoat material to a resin molded product 1 to form an undercoat film 2; applying a topcoat material to a surface of the undercoat film 2 to form a topcoat film 3; and thermally curing the topcoat film 3. A coating material that contains a base resin having a larger volume expansion coefficient than the resin molded product 1 and a granular filler having a smaller volume expansion coefficient than the base resin and a number-average particle size of 2 ?m or more to 12 ?m or less is used as the undercoat material.