Radiator shops use a special heat-conducting paint. I'd suggest stripping it down, and using a different type of paint. Also, just spray the surface, at an angle. You don't want to coat the entire core with paint, just the part that gets seen from the outside!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!