Being a semi-pro plastic modeler as well i could give you some advice as well.

Sanding with modelling sand papers should do the trick since they sand only the paint and not the metal. Afterwards apply an undercoat and then paint and a final clear coat of gloss varnish.

Model paints from humbrol/testors wont do the trick!
Your best bet is to get some strong paints that are meant to cover Lexan chassis on RC model cars. These are strong resilient paints that wont flake but they wont affect the rest of the paintjob on your bike. Car paints seem to be hard on a bike since the propellant and the nitrid components they include ,eat through existing paint.
Also if possibly use an airbrush.