I have polished out imperfections on plastic lenses of car instrument panels with semichrome polish, and lately Meguiar's Clear Plastic Cleaner #17. I've done plastic head light lenses, that were scratched BAD, with an assortment of wet/dry ultra fine sand paper, used for color sanding automotive finishes. If the scratches were bad, I'd start with 1200 grit and work up to 2500 grit. Finish it off with the # 17. They came out clear. The process should work on the dust covers.