PURPOSE: To provide the title undercoating material which gives an undercoat film having a physically roughened surface to thereby enhance the adhesion of an overcoating material applied by troweling; and to provide the title method using the undercoating material.

CONSTITUTION: A light-weight aggregate having a bulk specific gravity of about 0.3-0.8, a true specific gravity of about 0.7-1.5, and a particle diameter of about 0.05-1.0mm, e.g. one obtained by crushing and firing a tuff, is mixed with coating ingredients in an amount of about 5-30wt.% based on the mixture. Thus, the title undercoating material is produced which gives a film having a rough surface.