The latest manifestation of my marine insanity is a folding tri, a 1990 Dragonfly 25. The picture is a sister ship of the non-folding variety:

The important pieces are there. Good sails, trailer. Decent engine. Some electronics. It needs a lot of elbow grease.
For starters, there are bad spots in the ama decks. They are a laminate, although very thin. Balsa core of about 1/4". The bad spots are cracked on deck and flexing. Weight is everything on a multi hull, so I suspect that it trumped over strength in the design process.
I have searched around here, and found some discussion on deck repair, but I think because of the thin core I have a unique situation here.
I can remove the outer deck in the soft spots. The core will most likely come with it. Then I can try and re-core with balsa, although at 1/4" that sounds tedious. I can just fill with solid epoxy, or I can use foam. Your thoughts? And if foam, what kind and where does one get it?
Thanks.