I worked in a radiator manufacturing plant and in wholesale distribution of radiators and heatexchangers for 30+ years. My understanding is that radiators needed a coat of something to prevent corrosion and give them a finished look. The radiators we supplied to Peterbilt Motor Co before the mid 80's were painted a light gray. New aluminum radiators don't need paint. They are all one color anyway and don't discolor like copper and brass. The paint used was the least expensive and only a lite coat applied. Too much paint will insulate and slow down the heat transfer process. I think black was used to match the radiator support and inner fender color.
Steve
