We use to use a special paint at the CAT dealership I used to work at so we didn't clog the area between fins and so it would not peel. Here is the quote from Eastwood's site on why to use radiator paint.
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. This acrylic enamel factory-finish (85% gloss) withstands 300 degree F., sprays on thick enough to cover completely, yet thin enough to not interfere with heat transfer. Great for heater cores and air conditioning condensers, too! Each 12 oz net wt. can covers 6 sq. ft., most radiators require 2 cans for full coverage.
