If it's a work in progress, primer is fine... Although most of the time by the time somebody gets around to painting the primer---and metal underneath is junk. Primer offers no water or UV protection. When done as a final finish on the car,,,,,,,well,...it just isn't done til it's painted....

PS---and no, I don't need a bunch of 20 year olds telling me how things were done "back in the day"....