Clearcoat for Aluminum, Brass, and Copper

am using deep drawn aluminum cans with raised corners to encapsulate transformers on audio equipment.

Our intent is to apply copper and nickel plating to the .0625" thick aluminum cans and copper or brass plating to the bottom covers.

I need advice on how to finish the covers to provide a clear coat to prevent oxidation to the polished copper or aluminum.

The prime concern is how to finish the aluminum cans to achieve a dark black "patina" or patterned finish. The object is for each can to have exhibit a pattern similar to the damascene or folded swords.

I am not concerned with the amount of steps or costs required. Our volume requirements are low and our profit margins should absorb the costs.

Any advice on processes and sources will really help.

Thanks,

SIR,
WHY NOT TRY ANODIZE?
We do copper & brass colored anodizing on lots of electrical components.
Anodizing will also make your enclosures non-conductive electrically.