If the core is deteriorated then what harm can it do????? There is no structural wood in there and it is not a load bearing surface.
This is what I did and have been doing.
Ive got some soft spots along the decks just by the windows no doubt because they leaked in the past so I've been using caulk tubes from West Marine and injecting fiberglass resin into these areas around the wood block the chainplate backs up to and around the stanchion areas. These secure, now I move to the center section and inject spray foam then quickly screw in a screw to force it to expand outward.
AMAZING RESULTS! The pressure was enough to force the deck up to bow a little and now the decks are SOLID AS A ROCK! I can use a rubber mallet, jump on them, etc and they dont budge a bit!
I sealed up the windows by usinf a short piece of old rop and packing it in each window port gap all around then saturating it with resin. Solid repair there and finished with a coat of silicone.
Some pics of the deck....
