When I blasted the underside of my car using a pressure pot I discovered some things that really made a difference:
First, I used Aluminum Oxide. It's a very fast cutting aggresive media. Sounds dangerous to use you would think! Wrong! That IS what you want. Ideally, strip as fast as possible means that the there is less repeated media hammering the sheetmetal over and over. THAT is what creates sheetmetal warpage, not the type of media itself. Plus it gives a slight rough surface finish that the epoxy primer can really bite into.
Secondly, Since you are using a more aggresive media, you will really need to turn down the air PSI and media volume. This takes a little practice figuring out what's ideal. I slowed mine way down when doing tight corner areas underneath where I knew I needed to keep media exposure longer. Never stop moving the tip. 5-8 inches away depending on PSI, tip pointed at a 45 degree angle from the surface, media going away, stay in even slight overlaps. Never point tip straight at surface.
Thirdly, I didn't really need to media blast the outside of my car. The sheetmetal under the paint was all very rust free. So I opted paint stripper instead. IMHO, media blasting the outside should be done for cars that really show signs of heavy rust all over. Blasting a whole car inside and out means you will have a huge job later on cleaning media out for a long time after. It never stops coming out. I have just the underside done and I still see it coming out of the frame holes occasionally. I hear the best way to get it out of all little frame holes and nooks is to get a blower at high speed while it's sideways on a rotisserie. Then a blow tip followed up after.
Finally,
If you opt to do it yourself, you'll need a jumpsuit, hearing protection, a blasting helmet, and good air filter respiration. DO NOT use sand! Silica dust is very dangerous to breathe when sand breaks down into dust.
