I would think that the thickness of the powder coat may hinder the cooling capabilities of the radiator if you did it to the fins.
There was a cooling problem mystery thread a while back. His radiator was powder coated and hindered cooling. Might be different with just the tanks though.
well im going for a blackout finish on the truck and the rad stuck out too much.it looked too modern on an old truck.i can see it through the grille(that is black too.)i was thinking of just doing the tanks.ive sprayed the core/fins with high temp paint done the mounting brackets and the fan shroud.the tanks would finish it off.