If they still have their clear coat you need to strip it.
Then work from 400 grit wet dry, wet of course, to 800 (400, 600, 800) if you are going for mirror polish. If you want maximum reflection then go to 1000 or 1200 or even 2000 grit. Each successive step in grit must eliminate all the scratches of the previous grit.
Once you have them sanded then buffer/buffing wheel in a drill with brown polishing compound followed by white.