Without a doubt, the Permagrit tools are the gold standard for working with fiberglass and epoxy. They cut through it like butter.
Go Here:
https://www.fibreglast.com/category/s?keyword=permagrit
Well worth the cost!
Cloth backed Zirconia paper works great on composites and lasts just about forever. You can get it in various width/grit rolls to fit 1/4 sheet sanders and make custom length sanding blocks as long and straight as you need glued down to straight/flat boards. Wrap and glue to rounds for custom radius sanding. Wonderful stuff and the best we found for building a couple Cozy MKIV's. It was our go to abrasive and we ended up rarely using our Perma-Grit tools.
Is that the same stuff they make belt sander belts out of?
I've used them cut up to make longer sanding blocks and also in smaller pieces for and sanding inside curves.