Great work, Dan.
If you are using acrylics like I do. Before anything else, you can fpaint the portions where you want the chipped paint look with a thin layer of enamel gun metal or metallic grey. Let dry and cover with an acrylic top coat. To expose the metal portion or chip off the paint, you can rub it with a tiny, pointed sharp tool like a knife. You can also dip a cotton swab in Isoprophyl Alcohol and rub it in some areas to expose the gun metal or metallic grey paint. The first method will result to sharp chipping off patter while the second will result to a somewhat worn or violently scratch patter.
Good luck.