You probably want to fit a sacrificial anode to the radiator (or cooling system somewhere) so any stray current gets sopped up by the anode and electrolysis (?) doesn't corrode your aluminium radiator and engine.
Galvanic anode - Wikipedia, the free encyclopedia
Painting the radiator black might be good for appearance if that is your thing, but being black it will hold on to more heat than raw aluminium (or painted white!). Personally I like the look of an aluminium radiator.