Clean the current surface to remove any dust, dirt or old chipped paint.
Cover the old painted surface with a primer if it is degraded with a lot of chipped areas. Use a primer with zinc or zinc chromate to cover the radiator.
Brush any oil-based paint you like on top of the primer once it has dried. If you plan on using an aerosal spray paint, use a high-temperature paint.
Let the paint dry completely before running the radiator. Once it is dry, open all windows in your home and run the radiator a few times. Get that initial paint smell out of your home before you use the raditor for winter.