pieces and also the die-grinder sanding bands
we sell are both 80 grit. When professionals are using angle grinders they may use 36 grit, or even 24! The latter is definitely a bit course if hand sanding I find.
people seem really shocked by this answer, but it's definitely a lower grade grit that you need for sanding hypalon boat fabric.
Here's a photo of the sanding products we sell, with some hypalon which hasn't been sanded compared to hypalon sanded with an angle grinder. Hopefully the comparison is helpful and shows you just how sanded the hypalon fabric should be.