This is something I learned building models as a kid. I was going to post this on the radicons forum, but this is too simple and for people who dont kitbash but want to restore their old figs. There is a product called baremetal, i believe there are some other brands out there. Its basically like aluminum foil with adhesive backing but its not so sticky that you can pull off once stuck, you can easily peel it off (its made for model building). You need to handle it with carefully since it rips so easily. Apply like a sticker and rub out the wrinkles with a toothpick (take your time) this is also to help keep the foil to stick well. Here are some picks to show you.