The only way you're going to know is to dig in. Sand it, clean up the sanding dust, then wet it with a solvent like mineral spirits to make it shiny like a varnish job (or maybe put on a quick coat of varnish in a very small area) so you can see if you've still got the same issue, or if you sanded into any cracks or anything. It doesn't really look like glass fractures to me, but sanding it down is the only way to know for sure. Varnish is pretty elastic, but epoxy isn't exactly brittle, either.

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