I need some suggestions on how to mask the silver stripe around the outside of the horn button. The silver paint still looks new but the black plastic is a little discolored. I was going to mist some SEM Landau black onto the button but don't want to ruin the silver paint.

I tried pushing some tape into the groove and was going to trim it with a razor blade. The tape didn't want to stick though. It's not a hard tape line in there anyway.

The only other idea I had was to try and find a large O-ring that I could set in the groove. Any other suggestions?

Try some of the blue fine line tape that the auto paint supply stores sell. It's very flexible so it should be easy to work it into the circle of the cap. Bad thing is the stuffs not cheap. Should be able to get it in different widths.