Thanks for your reply John.

Yes - I will put a fill coat over the new fiberglass.

The original build did not have a fill coat over the fiberglass inside the cockpit. Impossible to know what went wrong of course, but my primary suspicion is that sand trapped between the ethafoam seat and the hull wore a path for water to get to the ply. There was no water staining or rot however. Maybe my butt was just too much for the plywood, BS1088 standards or not.