I'm no authority on radiator paint but I did question a radiator shop here in Yuma as to what type of paint that he used when I dropped off a radiator for repair, he assured me that their paint was strictly lampblack with a carrier an that that type of paint was not an insulator but a conductor..For what that is worth.... Bill
For the past forty years I have painted my radiator with the cheapest black spray can I can find. As Bob says you do not want to insulate it and cheap spray paint has little pigment in it. Spray from the top down because this is how a person looks at the radiator. You do not have to spray from the bottom up.