Saw an official m/b hubcap painting jig not unlike a logo-shaped cookie cutter on ebay a few years ago. It simply blocked off the tri-star area as well as the area outside of the painted circle's perimeter.

With mine I masked them off and spray painted (some shops can custom-mix paints -- like $20/can or so) after stripping the old paint. My efforts required some fine detail work w/ a razor blade to get things right. The wax method described above sounds like a better bet.

Regards,