Argh! Don't grind off your paint or powder coating to obtain a good ground! This will cause two problems down the road; first, this provides a great place for rust to start, which will over time, create enough insulating properties to cause a failure [which will always take place as you're passing the donut shop where the po-po is just pulling out of the parking lot to observe you with no taillights] and secondly, this rust, forming out of sight, will eventually present itself but by that time it'll be too late - repairs will be required.

The simple solution to this self grounding light fixture deal-e-o is to add a separate ground wire attached to the base/housing of the fixture which you run alongside the fixture lighting to either an earthen ground or a known ground in the bikes wiring harness.

I've found that bad grounds are the biggest problem encountered with vehicle lighting snafu's so please do yourself a favor with these types of fixtures and go with a separate ground wire to avoid any possible problems and repairs down the road.

=8^)