Here is what I am finding under some of the spiders cracks.
Most are just in the gel coat and I am grinding them out, but some are
into the fiberglass. The holes show up after I grind through the gel.
How should this be repaired?
Those areas need to be ground down into the glass and for best results all the cracks should be removed, then re-glassed, but that's not always possible. If you don't re-glass over them it won't be long before they're back.