I sand blasted every inch of my GMC sheetmetal, inside and out and didn't warp anything.
I used a small pressure sand blaster with a regulator set to 80 PSI. I kept the nozzle far enough away to remove the paint etc. and kept the nozzle moving.
Sandblasting should only be done for paint removal. If you think you are going to remove rust from sheetmetal to the point it is nice and smooth, forget it. If you blast away long enough, you are going to warp any sheet metal. You can clean off the rust but it is not going to smooth the metal.
Sandblasting is done to clean not heal metal.
