Good Question, I have always understood that radiator paint was a special paint that had good heat transfer properties. Some paints actually insulate which would hurt the efficiency of the radiator. From what I can find it seems that it is the “thinness” of the paint that is the issue.
Quick Google Search would seem to bear this out, but it is hard to find a definitive technical answer.
One source is
http://www.eastwoodco.com/jump.jsp?i...mType=CATEGORY but there is also a Canadian company that came up in the search, but I lost the link.