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I got a new set of half shafts from Corvette Central and spayed then with around six coats a Eastwood'd Diamond Clear. That was two years ago and they still look new. The car gets drive weekends,...
While I hate POR15, I think it's one of the worst products on the market, they make a product called Glisten PC. It is a cristal clear 2 part finish that absolutley will not yellow , and unlike POR15 it sticks to any clean surface. I use it to coat polished Aluminium , great stuff. Tough and durable even on Valve covers and other polished Engine parts.
Either seal them with several coats of good quality clear coat (if you leave them bare metal), or clean them well then shoot self-etching primer on the bare metal and follow with a few coats of good quality paint (semi-gloss black; aluminum colored engine enamel; etc.).