Using the wrong paint on a radiator can cause severe overheating problems. It's a heat exchanger, and painting it interferes with that process. If you decide to paint the core, use flat or semi-gloss black, and spray the fins at about a 45 degree angle so only the first fraction of an inch of the fins get painted. Don't let the paint penetrate very far into the core.
Jerry
I don't see why you say this. It's heat transfer you're looking for. Sure, if you use a heavy ceramic paint it will plug everything up and be so thick that you won't get heat transfer. However, a glossy vs flat surface will not change your heat transfer. I can say this because I have gloss black radiators in all my cars and they don't overheat.
