Hey Joe and thanks!
The hairspray technique consists of painting the model natural metal or whatever you want to show underneath and then spraying two or so (that's what works for me, you might use one or three) coats of hairspray over the model. Once this is dry then spray the green or whatever you want for the top coat. After this is dry take a toothbrush or a stiff paint brush, moisten it and then gently scrub where you want to remove paint. The hairspray underneath will peel up taking the top coat with it revealing the natural metal or whatever you have under that. Make sure the brush is moist and not wet or it will take ALL the top coat off.
I like it better than the salt technique since it's easier to concentrate the wear where you want it. It will also work like a reverse drybrush, taking the paint off the raised edges where it would chip first.
The salt technique is somewhat more random, I'm thinking of trying to combine the two and see if I can get the best of both.