Radiator shops where I did business used to use a special paint that didn't compromise the heat transfer of the fins. The other trick was to spray the paint at a 45 degree angle so only the front of the fin got any color. Most of the part of the fins that actually transferred the heat from metal to air didn't get any paint.
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!