Heaters have been painted with high temperature paint since the 1970s. Good heater paint is widely available and will withstand high heater temperatures. Spray cans of heater paint can be used to touch up your installed heater to make it look like new without removing it to a shop. Let the heater cool down first. Mask those parts not to be painted and protect everything around the heater from over spray. Most heater paint dries to the touch in about fifteen minutes. Colors are widely available too, so you can experiment by changing from traditional black to a more decorative color. How about a two-tone paint job?
Some heaters are factory enameled, finish that cannot be added later. Enamel is very tough, even under heat stress, but can be damaged by chipping. Touch up and enamel filler kits are available from heater dealers.