First, as most are suggesting, such deck repair is not simple job.

However a friend of mine had already purchased an Ericson ('60s vintage)
before he discovered it had spongy cored decks through out. This, in spit of
the fact that he had a complete survey prior to purchase that didn't mention
the problem.

So far, I'm surprised no one has suggested repairs from the outside. This
is how my friend did his. Saved having to take the interior apart and all
the related glass mess inside the boat. My friend was living aboard at the
time. Also eliminates the need to do overhead glass layup.

With grinders he cut out section of the deck glass, from topside and just
down to the cores (ended up being 100% of the total deck area), cleaned out
the old rotten/soggy core material and rebedded new core foam to the old
interior glass work.

Since the deck cores generally
--
My opinion and experience. FWIW

Steve
s/v Good Intentions terminate just short of the bulkheads, each section that
he removed ended up running from bulkhead to bulkhead.

Once the section of core material was inplace, he layed in the first couple
layer of mat and roving, leaving the final layer to be accomplished after he
had all of the adjacent areas recored and glassed in.

When completed, smoothed and faired, he applied the non-skid patterns from
sketches and measurements. I think he used crushed walnut shells, set in
resin or paint.

He estimates that he added a few hundred pounds to the the deck weight but
no more than the deck weighted with all that water in the core.

BTW, this fellow is a lawyer and this was his first boat. The surveyor was
arranged by the broker.. My friend took the surveyor, the broker and the
seller to court but the judge throw it out.. Kinda like "Buyer Be Ware" when
purchasing a boat that old.