I agree with adtkart. I've probably got the same Sears sandblast kit.
I use "blackblast" for the medium which is a product packaged in Minnesota and sold here in Wisconsin by Menards (a store just like Home Depot). I've done my entire car, frame, fenders, hood, trunk, entire body, etc and never had warpage. Right now I'm blasting the Ford rear end.
I bought a cheap hood to wear which keeps the sand off my head and protects my eyes (although I also wear glasses under neat) and I also wear a breathing mask so I don't breath any dust.
Have fun with whatever you do because it's all part of the buidling process.