Dual coated metal substrates and methods of making the same

A method for coating a metal substrate, said method comprising steps of:

• (I) applying onto the metal substrate a first liquid coating composition comprising, in an aqueous liquid medium, an intimate mixture of:
• (i) a hexavalent chromium-providing substance, supplied by about 80 to about 100 weight percent chromic acid and providing above 5 but below about 100 grams per liter of chromium, expressed as CrO.sub.3 ;
• (ii) above 50 but below about 500 grams per liter of liquid medium of pulverulent metal selected from the group consisting of zinc, aluminum, mixtures thereof and alloys of same, said first composition having a weight ratio of chromium, expressed as CrO.sub.3, to pulverulent metal of between about 1:1 and 1:15;
• (iii) below about 50 volume percent but substantially above about 15 volume percent, based on the volume of the total liquid of the first liquid coating composition, of water soluble organic liquid substance that maintains liquidity above 100.degree. C. and is selected from the group consisting of tri- and tetra-ethylene glycol, di- and tri-propylene glycol, and the water soluble C.sub.1 -C.sub.4 ethers of all such foregoing glycols, diacetone alcohol, the water soluble C.sub.1 -C.sub.4 ethers of diethylene glycol, and mixtures of the foregoing;
• and
• (iv) above about 0.0005 volume percent based on the total volume of first liquid coating composition, of dispersing agent;
• (II) heating the substrate and the liquid coating formed thereon in step (I) at a temperature, and for a period of time, sufficient to convert the liquid coating formed in step (I) to an adherent, water insoluble first solid coating on the substrate;
• (III) applying onto the surface of the first solid coating formed in step (II) a second liquid coating of an acidic aqueous composition comprising dispersed resin solids and an activator, which acidic aqueous composition forms on a metallic surface a resinous coating that increases in thickness the longer the surface is contacted with the composition, to form an uncured autodeposited second liquid coating over said first solid coating;
• and
• (IV) curing the uncured autodeposited second liquid coating formed in step (III) to form an autodeposited second solid coating bonded to the adherent, water insoluble first coating.