PURPOSE: To polish a resin painted surface of an automobile or a plastic surface extremely well into a mirror surface without generating a buff trace by combining a polyvinyl alcohol sponge of appropriate maximum water-holding coefficient and a fine polishing grinding grain together.

CONSTITUTION: A polyvinyl alcohol sponge of 800-1,500% of maximum water- holding coefficient is installed on a rotational polisher as a buff material. Water is absorbed in this sponge, while a dispersion material of a polishing grinding grain whose average grain size is 5μ or less is held on the surface thereof, so as to polish and finish a resin painted surface or a plastic surface into a mirror surface. The frictional heat generated between the sponge face and the polished face is absorbed by a great quantity of water held in the sponge, whereby increase in temperature of the polished face is prevented. The resin painted face of the polished face is not softened thereby, while a polishing damage called as a buff trace is not generated, and a perfect mirror surface can thus be obtained.