In the "old days" places like Sheib, Macco and Miracle would typically assign a minimum wage kid 15-20 minutes to prep a car (basically try and clean off most of the bugs and run a scratch pad over as much of the surface as he could in that time) before they masked and shot a car.
We had a car painted way back then by one of these - paid extra for a better prep and primer. When the paint peeled we could see no primer underneath the top coat. We were kids so the salesman tried to give us the "clear primer" line LOL That didn't go over at all.