After the "metal" paint has dried fully (that is important) take some basic white toothpaste (the cheapest you can find will do) and smear daubs of it across the areas you want the "metal" paint to show through, ie: edges and other high wear areas. Then paint the final coats as you usually would and once those are fully dried (again, important) you just take a soft, damp, cloth and wipe away the toothpaste, revealing the paint below. keep wiping with a damp cloth until all the toothpaste is gone. I would then dry brush some black or gunmetal grey paint across the metal paint to give it the worn look, and then you can apply a sealing coat of clear lacquer, either matt or gloss, depending on your preference. I hope this helps!