PURPOSE: To obtain an anodic oxide film excellent in durability and adhesion of a synthetic-resin coating by completely sealing the anodic oxide film on the surface of an Al-base metal and washing the film with water and then with hot pure water obtained by ion exchange before coating.

CONSTITUTION: The anodic oxide film applied on the surface of Al or Al alloy is completely sealed with a sealing assistant to improve its durability. The surface is washed with water and further showered with water to remove the deposit such as the sealing assistant. The film is then dipped in the hot ion- exchanged water of specified purity, hence the surface is stabilized, and the adhesion of the coating is improved.