I got some paint from
Eastwood Co. that is made specifically for that... I haven't had a chance to use it yet though. Here's the description:
Standard engine paints are too thick for radiator use because they can interfere with heat transfer and block air flow between the cooling fins. Also, standard paint tends to flake off radiator tanks. Radiator Black helps solve this problem. Durable Enamel formulation resists temperatures up to 250ºF, gas and chipping.