Per the advice of Aerosport Products (Geoff), I spent the day applying USC Icing Polyester filler to the rough interior surfaces of the fiberglass cabin top, and then sanding most of it away to gain a smooth surface for painting.
The Icing material mixed easily but has to be applied quickly as it sets up in just a few minutes. After about 30 - 40 minutes it was ready for sanding. Mixed up about a 4" x 4" layer of Icing and applied it, then mixed a second batch and applied it as well.
Started with a random orbit 5" disk sander with 100 grit pad to take off the high spots, then moved to hand sanding. Started hand sanding with 80 grit paper, using both a very small paint roller refil as a form to hold the paper, and a piece of 3/4" pipe insulation for larger diameter sanding block, then moving to just hand sanding in places.
Most of the time was spent sanding, but the result is a much smoother cabin top interior, which will make for a much nicer interior finish when painted. Still have at least another day of this work to go.
