I can’t speak on its availability stateside but in Canada there is a primer marketed by Dominion Sure Seal called Flex Fill. You can pick it up at Canadian Tire or your local paint or auto parts jobber. I use it on all my bodies before paint and it always works well. Builds enough to do some minor leveling, comes in a large size can and can be top coated with lacquer, enamel and automotive urethane without problem.