One of the customers of my former workplace earned money for flying lessons when he was much younger, by polishing privately owned warbirds at the airfield by where he lived.
The conversation came about when he stopped by the shop one day and noticed I had an aluminum K-9 partition removed from a police cruiser and was in the process of polishing it.
He also gave some good tips on how to rub the finish out without leaving "swirls", so it seems that the aircraft are bare (non clearcoated) or at least were in his time.