Sand wet only with a drop of dish soap in the water. Personally I wouldn't touch it with anything coarser than 1500. Sand the whole surface, not just the scratches. work up to 2000 grit or finer. Hand polishing just doesn't cut it, you need to machine polish it to get a crystal clear finish BUT any heat will melt the plastic so nothing faster than a drill and not too much pressure - let the buffer and compound do the work. I put a small buffing wheel in an air die grinder and turn the pressure down so it turns slow - more manageable than a drill !! You can use the stick polishes that Harbor freight sell - white or red.