2 things, if you're talking about exterior sheet metal, any sand or slag media will tend to warp the metal quickly. Use media like that on non-sheet metal parts. And second, don't use silica sand to blast with, at all. Read up on silicosis. Silica sand breaks up into too fine of dust and causes respiratory issues (even moreso than breathing other types of dust does). In place of sand, use coal slag, something like Black Beauty or Blackjack. There are a few who will maintain that it's okay to use sand... it's just not, don't do it. Save it for the sandbox.
