I did some fast goggling, and came up only with a good reason to paint your radiador. Corrosion. Which, in a normal house enviorment, i dont think it's critical .
Nevertheless, for what i read, Electrophoretic coating is the way to go, it gives the thinnest layer of paint, thus not affecting performance very much.
source :
http://www.cuprobraze.com/cuprobraze...s/coating.html (ok, not the best source...)