I'm rather sure my deck core is rotten. I know that the best way of fixing this is to cut the deck skin off to remove rotten core, replace the rotten core with fresh wood, then re-attach the deck skin. That sounds like way too much work for a boat that I got off of craigslist for $1200, and honestly, it doesn't sound very necessary.

I know a lot of people think they can put unthickened epoxy into holes drilled into the deck, but because epoxy runs all over, I've heard this isn't very effective.

What if I pumped expanding foam from TAP Plastics or the aerosol foam insulation from Home Depot into a grid of holes drilled into the deck skin? Once each hole is filled, I can then fiberglass over the holes. What's wrong with that? If such foam is too weak, what about epoxy thickened with microspheres? Sounds like it makes sense, but evidently nobody on the interwebs has done this and posted about it, so I'm wary of trying to do it myself.