One of the tell-tale signs of pits/rust coming through from the back side of vintage sheetmetal is a dark grey circle as compared to the rest of the metal surrounding it. Here's a sample of rust coming through from the inside, some are far enough along for holes..
Your paint materials and application labor are going to be the largest expense in these repairs, it would be a damn shame to scuff and paint only to have pits come through within a year or two. Fix the rust while it's cheap, ie: before the paint goes on. Without stripping to bare metal you will miss the opportunity to better see where the rust is going to come through before it happens.