I once tackled the plastic (plexiglass?) front windshield on my Cessna 172, which was pitted. I started with block sanding with 600 wet or dry, ending with 2000, 4000, and polishing with Maguire's "Mirror Glaze". Lots of work, but the results were perfect. The same can be done with any plastic canopy. I think the secret is using a block to sand, with plenty of patience. Can defects be detected from 6 feet away?