24 years of experience in the business, and 15 years before that as a hobbyist. I have seen it many times, people strip a car and leave it exposed to the elements, even in a closed building, flash rust starts immediately. What happens when you sand steel...it makes scratches. Guess what grows down in those scratches? Rust. The shop I am in now did an 85 Mustang GT about 4 years ago, it was a typical faded porous 80s paint job. Paint shop DA'd the whole thing to the metal, primed it with Chroma Premier, painted with Chromabase and clear, comes back all the time with blisters in places like the valley on the hood where the style line is. It's almost impossible to sand out all the rust on a car that has been sitting exposed for months/years. You will notice I said a blaster that knows what they are doing. This does NOT mean the same guys that do construction equipment, industrial machinery, trailers, etc. You need a blaster that uses the proper media for the job. Sand is never the proper media for automotive sheetmetal. A good blaster isn't cheap, a cheap blaster isn't good. Strip a panel, prime right then, preferably a DTM epoxy, or at least a good etching primer like Variprime, followed by a primer/sealer....but epoxy is the preferred choice. Is it more of a PITA and an inconvienence to do it this way, sure it is. Time is money. Is redoing a complete a PITA, you bet it is and the way materials are priced, nobody wants to pay for that. What kind of a guarantee/warranty are these guys going to provide? Unless you live in the most arid climate in North America (and as I recall from living in Iowa with family in WI for 25 years, it's not), I wouldn't even consider just sanding and spraying,unless it's a flipper. And, we haven't even addressed the subject of NOT having a standard collision shop take on a job like this.