if you must paint aluminum , clean it with very strong cleaner or aluminum cleaner the sand it with a gray scuf pad, green will work also but dont sand as hard then prime and paint
Not sure if it is still available but in the late 80's there was a 2 step process. 1st was a cleaner/ etcher, 2nd was a primer just for aluminum after shooting it you could just sand, prime w/standard primer, sand, and paint. It worked very well, but judging by how well it worked, it probably caused cancer in lab animals "LOL" at least in So. Cal..
Yeah, you could get it anodized black, although this would require it being removed from the car and being out of you hands for a period of time. Eastwood make a radiator paint that is in a can that is thinner than regular paint if your going to paint it.
use a good lacguer paint, you can usually get it in spray bombs at the parts store, you can put it on pretty thick on the tanks but just spry it lightly on the fins and tubes, if you get it too heavy on the fins and tubes it could create cooling problems especially on an aluminum radiator.