We are not in any disagreement I assure you. It can be done, it is done, I know that. My point maybe wasn't made clear. It CAN cause this damage and many times will.
I have seen little home sandblasters do it, of course how big these "little" sandblasters and or the compressor used is the issue. The fact is, I have seen it MANY times. You have either been lucky or used great care with common sense (I choose the later). Not everyone has the skills to pull it off. For this reason I aways recommend against it.
The problem is as I said this causes some of the most severe damage I have ever seen. I have been around this stuff my whole life and I have had darn hard time doing ANYTHING with some of the panels I have tried to repair. I am sure someone of Randy's talents can repair it (I should say, "I assume", there are some things that are beyond hope) few home hobbyest with the hankering to get out and sand blast have the skills to repair the damage it can cause.
As far as the repair, "how to tell if it is the inside or outside that needs shrinking?" that is a darn good question and the hardest part. How can you tell if both sides were blasted? If only the outside was blasted then the outside is what is stretched. If both sides were blasted, then both sides are stretched at different places? I don't know how you can find where to do the shrinking. I can do wonders with a crash damaged panel, but the sand blasting damage I have seen is just darn hard to repair. I have usually ended up "getting it close" and stopping it from "oil canning" and then filling the darn thing body filler.