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
Good point. When I had my custom Frankland Quick Change built at the Frankland factory They put a special black paint on the aluminum bells that they said would make it actually run 20% cooler. It's a flat black paint that is not very durable but it's not an area where it gets a lot of wear.
