Falconvan - I agree that it comes down to technique and experience and one other thing; good equipment. I doubt that Vara4 would get the results he wants with a $15 flea market blaster like mine. Also, it takes a lot of practice (that's where experience come in) to successfully blast sheet metal without warping it. I have been doing body work off & on for 46 years and I have seen a lot of body panels ruined by overzealous sandblasters. Your advice to him is good: find somebody who has the experience and has done others he can look at. Otherwise, I stand by my original opinion, don't do it.

Vara4 - Firebird77clone is correct; door jambs, floor pans, firewall, and other structural parts that are heavier gauge steel are usually not a problem. It is the outer body panels that are subject to warping and expanding. Blasting the inside of your roof is the same as blasting the outside; it will end up with so many waves you can go surfing on it. If the inside of the roof is going to show, I would brush on a coat of Ospho, let it sit at least 48 hours, and finish it (prime, scuff sand, paint, etc.). If it's going to be hidden by a headliner, I would brush on the Ospho, let it sit 48 hours, and spray on a coat of rubberized undercoating.