Warpage typically won't be a problem for those of using the small sand blasters that run off our home compressors. They simply don't have the horsepower to do the damage.
Kind of like comparing a garden hose to a fire hose...
However if you rent an industrial compressor or have access to one that's where you can get into trouble. I rented one to blast the body on my Model A. The compressor was a V-8, with 4 cyliders used as a compressor. It had enough power to cut holes through thin sheet metal.
Here's a photo of the Tudor getting naked:
http://i645.photobucket.com/albums/uu176/lakeroadster/1931%20Ford%20Tudor/0321991.jpg
