I must add to this thread as some people made some incorrect statements.
I have sandblasted 3 cars and not found a single distorted panel due to sandblasting. I even tried to warp a panel and could not.
The secret is to understand what you are doing.
I run a pressure blaster at 40 psi.
I open the sand mix valve on the bottom till I can just make out that sand is in the stream.
This gives me low energy impact. I have a small amount of particles traveling with moderate amount of energy. It slowly takes everything off.
On the larger sheet metal panels I try to keep an angle to the work, though my testing proves this is not needed. I have even blasted thin French car metal with no distortion.
We started blasting our own parts when we paid to have a guy blast some Model A parts. They all came back ruined. Of course, they were done with high pressure air and a large quantity of sand in the stream.
For safety I used an air fed hood and a respirator. Sorry guys you only live once and it only costs a few $$ for such protections.