I used to work in a tractor factory on the small parts paint line, and we used to take our cases in on nights and get them dipped in the caustic baths they had there. It strips the metal down and removes any rust etc. We then used to stick them on the line and get them sprayed, which was great as long as you liked Case Tractor red or black
Anyways, ramble over, you can get the cases dipped at some metalwork places if you do some ringing around. I know there are a few places here where they will do it for a small charge.