FORMATION OF COATING FILM

PURPOSE:To make a polyamic acid resin coating film free from pinholes and sag and to form a heat resistant coating film having superior corrosion preventiveness and reliability by immersing a base material with the polyamic acid resin coating film having heat resistance formed by electrophoresis in a mixture of a specified org. solvent capable of dissolving the polyamic acid resin with water. CONSTITUTION:An Al plate to be treated is put in a stainless steel vessel filled with an aq. polyamic acid soln. for electrophoresis and electric current is supplied between the Al plate as the cathode and the vessel as the anode to form a polyamic acid resin coating film having heat resistance on the surface of the Al plate by electrophoresis. This coating film is liable to undergo deterioration in characteristics owing to pinholes and sag. In order to prevent deterioration, the film is immersed in a liq. mixture prepd. by mixing an org. solvent having >=100 deg.C b.p., capable of readily dissolving the polyamic acid resin and miscible with water, e.g. N-methyl-2-pyrrolidone with water in 20:80-95:5 ratio and the film is dried and heated. A heat resistant coating film free from pinholes and sag as defects and having superior corrosion resistance is formed.