[painterblaster]

While having a conversation with car restorer about warping sheet metal resulting from media blasting, he showed me a '55 Chevy trunk lid he had blasted inside and out. Where the "cut-outs" were in the inner structure that revealed the back side of the exterior sheet metal, the panels were noticeably warped, distinctly only where the exterior metal was blasted on both sides. He said he had never warped a panel when blasting only one side, and came to the conclusion that when you blasted both sides, the temper was taken out of the metal, resulting in the warpage. I'd love some input and opinions from the pros on this forum.

 

[mrcharles]

Sounds more like user error. Cars are blasted inside and out all the time.

 

[Betterblast]

Mr Charles is pretty much correct. There is always some chance beyond your control, but experience counts.
You have a couple different things that can be going on. First, just too much dwell time with maybe too much psi or too close. A finer media is more gentle. The metal should be tight against the supports in the back or you can out in a shim to create support. You should not be able to pop it in an out with thumb pressure. The back is always more difficult because it is curved away. The blast wants to pull metal towards you. So when you blast the back it is asking the metal to pull away from its natural contour and you can get that "oil can" effect where it pops in an out. It doesn't matter if the out side was also blasted as far as I can tell.