sand blasting

My understanding is that the factory dipped these bodies ion a vat of primer, up so far. Was it about a foot or so above bottoms of doors???? I forget.
Years ago I have sandblasted undersides of cars or chemical dipped hoods. The part that always worried me was getting primer back onto those "cleaned" parts.
I have always said, most of our cars sheetmetal rusts out from the INSIDE> But I have never lived in the damned rust belt either!