Below is a picture of my roof after sandblasting, priming and then spending a considerable amount of time and effort making it right again. Trust me, its not something that you want to see. It all depends on the media that you use and the amount of pressure you throw at it as to the severity of the warping. It would be difficult/impossible to get in close enough to see it warp as you blast it, you can barely see it while blasting. After blasting its more of a feel thing. When I did my quarters I stood back 4 feet and took the paint down to the primer and stopped. Sanded the rest.