You can wet-sand starting with +/- 400 grit (or even 220 if they are really bad/pitted), and work your way up to 1500/2000. Once you are happy with the finish @ 1500/2000 (should still be a bit dull), clean with IPA and shoot them with a a few coats of a quality 2K UV clear coat. Once cured, wet sand again w/ 1500/2000 to remove orange peal from the clear coat, then polish . If you just wet sand and polish the plastic without using a clear coat, they will haze/yellow again in no time. Also, beware this is a fairly messy job!