Epoxy for coating a radiator?

The color of a radiator or an engine doesn't make any measurable difference in the cooling ability. Color is only a reflection of light.
In a dark engine bay, there essentially is no color because there is no light. And the heat comes from inside the engine and inside the radiator, and the only color there is either rust or whatever color of the metal is. Paint is only there to prevent corrosion and look pretty. It really serves no purpose in cooling other than to put a slight layer on the outside of the metal where you really wouldn't want it in all honesty if you can get away without it. But again the difference and the effect is very minimal. The color of an engine or radiator is completely irrelevant. Your car is not going to cool better or overheat if you paint it a different color. Even if you had an engine sitting in a rail dragster out in the sun, it wouldn't matter much. If anything you would want it painted a very light color to reflect the heat not black to absorb it. But again that would only be if you had something on the bleeding edge and were borderline overheating already and you needed every last little possible advantage. In any kind of realistic scenario the color matters not one bit.
You might could figure out that there is some small difference in the heat blocking of certain colors when they are sprayed on metal due to the pigmentation or the solids in the color, but that's due to the formulation, not the actual color. And the difference is so minimal that it almost can't be measured and makes absolutely no difference in an automobile.

As far as radiators go, Copper/brass has the best heat transfer of any radiator. But it is not very strong. Aluminum radiators, some people think they cool better.. pound for pound they do not. But, aluminum is a lot stronger and you can make the rows (tubes) bigger, and therefore get more airflow coverage to more than overcome the difference in heat exchange. If you could make copper/brass that big and have them to be decently strong, that would be the best combination, but you just can't. So they have to be smaller with more rows. That's why you can replace a 4 row copper/brass radiator with a 2-3 row aluminum and it cools the same. The rows on the aluminum one are a lot bigger, because it's stronger and they simply "can" be.

Whatever paint you use you want the absolute thinnest coat you can have on something that is cooling. Engines and radiators cool by convection, not radiation. Thick paint would actually insulate a bit...but again, the effect is still negligible, but if we're splitting hairs, then as little paint as possible.
 
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Most OEs these days leave their cooling components in bare aluminum, unless they put a little black on the outermost component so it's less visible behind the grille. But even this is becoming rare as far as I can see. So it seems they may have done the math and decided that the "juice isn't worth the squeeze" as far as black increasing cooling efficiency. I've never seen any scientific studies on this topic though, so my observations should be taken as strictly anecdotal.
 
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