I've decided to try a unconventional way of fixing some wet spots in my deck. I wanted to avoid weakening my deck too much, so I thought instead of totally detaching the top FG layer, I'd drill out some big holes and poke the wood out from there:

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Indeed the balsa was totally wet and had mostly detached from the fiberglass. My next steps will be to remove the wood below the rims of the holes, maybe with a wire brush attachment for my Dremel or drill, dry it some more, and then fill it all with epoxy. Maybe try to thread some pieces of FG in there, too. I'd cap it with the round deck pieces I saved, fill the gaps, and fair it.

What do you think?