So I bought a used DeWalt 421 5" orbital sander. With a 80 grit disc, I ground off the gelcoat and into the resin. I didn't see any fibers after a little bit, so I hand filed down in one spot till I started getting into white colored stuff again. The pic here shows that. Directly below this whitish area in the picture you can see the original crack, partly filled with white gelcoat dust...