There is some very important information left out of all these posts.
What pressure, relative sand density and angle was being used in each case?
I have done 3 cars, yes full cars by sand blasting. I have not been able to find a single panel with warpage. On the other hand, we paid someone to sandblast and the panels were warp to crap.
What is the difference?
Well the guy who we paid blasted at 100lbs and with the sand valve wide open. You have a lot of particles moving at high speed. Lots of energy to move metal.
We run a Model B engine with a compressor head. It runs at idle and we have it set to give about 40 lbs of pressure at the pressure blaster. Then we only crack the sand valve open until we just start to see sand coming out. At these settings you must be patient, but on the other hand I did some thin Frence car panels with NO warpage (I also kept the noozle at and angle to the surface).
Now if you open the valve up more than you are increasing the amount of energy your beating the panel with. So even at 40lbs you might be able to warp a panel if the sand valve is wide open. I never tried this to see what would happen.
So yes if you want to be impatient and blast at high pressures with lots of sand in the stream you are gona warp some panels.
You can safely blast sheet metal if you pay attention and run lower pressures and keep the sand valve minimumly open. This will take a lot longer and is more effective (or faster) at removing rust in the pits than other methods.
So will those who swear that blasting will always warp panels because it has happened to them, speak up and truthfully tell us the whole story. What pressure were you using and how wide open was the sand valve.