WRT the dustless blasting, the main complaint I have heard is that using water as a propellant invariably leaves collections of "slurry" in those same rust prone areas that you are trying to clean, making it a moisture collector that is now going to start the very rust we're trying to get rid of. So I would not recommend using this process on something that cannot be thoroughly cleaned out..
Speaking to (dry) media blasting and the damage that may occur, most times the damage occurs when someone is blasting from the inside and working around the "openings" of an inner structure. Damage such as this follows those openings exactly, and kinda verify that it was indeed sandblast damage. When someone is blasting along cleaning up the inside oblivious to the fact that nothing is hitting the panel underneath structural braces, the "stretch" occurs only at the opening. The peening action starts the stretch on the same side that the media hits, so it stretches INWARD toward the media source.
Here is a media blasting damage "study" I did, and various methods to correct.
http://67-72chevytrucks.com/vboard/showthread.php?t=647269For heating methods, Robert kinda shys away from torches for sheet metal shrinking because I like the slower sneak up on the fix approach. Faster methods tend to have me sliding past the exit ramp sideways, and then I need to stretch again.
My preferred method is using the heating tip on a dent puller, trunk lid upside-down in the case of my study, so we have the stretch pucker upward, and then use the heating tip on the high(est) spot. By shrinking while the weight of the dent puller is against the panel (but don't push, you won't need excessive force) you can actually see the panel move down/outward as the shrink occurs. But it boils down to using the "shrinking" tool you have, and what works best for you.
The panel really doesn't care which side heat is applied, it will shrink regardless. But in my experience, just having that little bit of static weight of the shrinking tip against the inside, it seemed to restore proper panel crown more quickly in approaching the repair from the stretched side.
So, with all that said, prevention being an ounce of cure, etc, how do we prevent that damage? In equating sandblast damage to a hammer swing, Media size = hammer size. Air pressure = hammer swing velocity/force. So to make sure we aren't using a sledge hammer, the media size should be medium to small, and to reduce the force of the hammer blow, air pressure as low as possible. Next, if media blasting the inside of a hood/trunk lid/door where a structural member (with openings) is against the inside of the outer skin, block off those open areas so you can media blast the normally irregular surfaces of that structural member without media going past the access holes where it strikes the back side of the outer skin. Then follow up that with abrading the back side of the outer skin with a DA and 80 grit.
