In reply to ewanjp:
Use car paint on metal gear. Clean gear thoroughly, spray/apply paint and make sure it gets into the markings/knurling. Even when most has rubbed off, there will still be a small amount left in those awkward letters and grooves. When down to that level reapply. Cheaper and more wear resistant than nail varnish.
Did mine when gear from the group started to 'disappear'. Everyone had tape on theirs in a colour scheme devised by the club. Peeling tape and cleaning off the residue is easy, removing all the paint from awkward places takes much more effort. As soon as I used paint my gear stopped 'disappearing', although others still had theirs 'disappear'. You can't always trust friends.