Are you going for a worn all-metal piece? Or like something painted with scratches w/ metal beneath? If it's for like a replica weapon, base coat it in black, use a decent silver spray over the object. Spray the primary paint needed over it. At that point, what I do, is take like a sanding sponge, and gently take it to the edges and such that a real world object would get in time.
Another method I just learned from Sean Bradley's tutorial, is to put the silver base coat, and then take like one of those disposable brushes you get at Home Depot, using toothpaste as a masking fluid, brush along the areas you want to appear "weathered," then spray your primary coat over it. After that's dry, wipe off the toothpaste, and it should reveal pretty realistic scratches!