Home made fan shroud.

RichB said

You need to use the total surface area of your radiator for the radiation calculation, which is much more than the frontal area. It will include the tanks, the tubes, and the fin surface areas. Color may not make much difference in total engine cooling but black surface color is best for the radiant cooling.

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You're right, I did only count the front of the radiator and not the whole thing. That might get you a little more than double: the back surface plus the tanks and sides. And yes, black is the best for radiant cooling. But I wouldn't expect the surface area of the fins to make any difference, since a material is just as efficient in emitting as it is absorbing, and when one fin emits some radiant energy, the next fin over that it's facing is going to just reabsorb it.

I was just trying to see for myself how much of an effect the radiant cooling has. Since the airflow is hundreds of times more efficient at removing heat, I don't think it makes any practical difference.