Some are red oxide primer. That is what the bodies all had on them. Most were primered gray over that but I have seen some that were not. The gray was never that thick and usually was the cause of the paint flaking off. When ever you would see a car with the paint coming off it was usually down to the red oxide
My first 70 Swinger in 1972 (FC7) had the peeling paint on the trunklid. IIRC, I read a TSB about the primer being the cause. But, only my trunk had the problem.