Rod at Blast Off using plastic media for large panels that might warp and uses aluminum oxide for sturdy areas, especially those with rust. I didn't end up with any warpage that I can tell. Of course, I didn't have much in the way of big surfaces and the roof already had an oil can spot.
My logic behind cutting the quarters off before blasting was that 1) I knew they were trash 2) having them off allowed Rod to blast the wheel well and other parts that would be accessible 3) less media would be trapped in the car 4) I can easily prime the clean metal all over prior to welding on the quarters. Worked out well.
Note that I had already replaced the floor and trunk pans. The aluminum oxide cleaned them up nicely and I just have to fill in some pinholes in my welds, dress the underside, and I can prime.
The blasting uncovered a nasty roof. Lots of pits and even a few pinholes. That oil can spot is going to require some shrinking to fix. BUT, there was this totally rotted out Challenger sitting down at the shop for Rod to blast. I'll take pics when I go down to pick up my other parts.
