BV - I have only done a few vac canopies, but they were all formed with the frames raised. That left a corresponding relieved area on the opposite side, (the interior.) If the material is actually thick enough to sand down the frame, and still leave enough for the surface to remain intact, then you'd be OK. I guess it would depend on how prominent the frame was formed, if just lightly raised maybe enough material would be left to let you sand it away.
For polishing I use Novus, after the very fine polishing pads, 6,000 to 12,000 grit.
Good luck with it, hope it works.
Patrick