For all of my battle damage (armor and helmet) I did the following:
First, I sprayed a few coats of primer, followed by 5-7 coats of Rust-O-Leum "aluminum", depending on the item.
Then I took a roll of "perfect release" masking tape (it's the purple kind that has less adhesive than the blue kind) and drew the shape of EACH scratch/scar/mark on it one by one.
As I would draw a scratch/scar onto the tape, I'd then cut it out with an X-Acto knife and apply them to where they went, using the Fett reference CD for guidance (my helmet had 93 individual pieces of masking tape on it by the time I was done!).
Finally, spray on a few coats of the top color, let dry, then remove all of the masking tape. Looks very real, since the silver/aluminum/chrome is not applied topically, and the top coat is actually on TOP of it. Very time-consuming, but really great results, plus with the "drawing on the tape method", you can cut out a more precise scratch/scar than you can usually paint one. 
Oh well, just something I came up with. Your mileage may vary. 
