: Howdy! I'm doing some modifications to a glass boat. It is old and seriously
: worn. No idea how old, what it is made of, ect, but it looks to have sat
: I'm wondering if I can just sand whatever resin it is down and bond to it
: with epoxy? I've gotten conflicting answers from credible experts, which
: is why I'm asking more here. Someone suggested that if it's vinylester
: resin, the epoxy won't bond.

Epoxy will bond to vinylester, but leave a roughly sanded surface for the best bonding.

: Here is a photo of the inside of the boat, is that kevlar? how can I tell?

The yellow color looks like Kevlar to me, it could be that the resin has yellowed over the years, but the weave also looks like something you would expect of Kevlar. If when you sand it, it gets all fuzzy and hard to sand you will know for sure, but I'm willing to bet it is Kevlar and you will hate sanding it.