I used to marbleize the old Gerber 4B's, actually had people pay extra for them, lol. We clean up machines all the time, our cabinet holds paint thinner, acetone, different variations on Vim, rapid remover, alcohol, alcohol/water mix and whatever else it takes. We also get custom matched and factory paints, plus we buy a lot of new covers, usually only when a keypad is affected though.
We usually do not do anything unless we make the decision to totally refurb though. Clean covers on a beat machine look almost as bad as groddy ones.
Craig