i would remove the radiators, take them to an auto radiator shop and have them cleaned in their dip tank to get the interior cleaned. it the radiator shop cannot clean them, take them to an automotive machine shop that has a dip tank that is used to clean engine blocks prior to machine work, have them clean the radiators. this should remove al rust and paint from them. you are now ready to paint. for best results they should be painted with an enamel paint.