\It really depends on how much is good and how much is soft in any
amount.  If there are any more than isolated soft spots be prepaired to
pay out some bucks or spend a year in hell.  Otherwise, I would look for
another boat.
Soft decks means moisture in the core.  Real soft decks means rot.   You
will need to rip down to the lower skin back to solid core and then
another foot or so of top skin only.  Let the edges dry out, recore and
reskin and then fair out to the original deck.   You could core with
plywood but the added weight will raise the COG and make the boat a
little more tender.
Also if the rot is extensive you better make sure how the cabin top is
supported.  On many production boats the side decks provide most of the
support for the cabin.  When you remove that support the cabin top must
be held in place by temprary bracing and tied carefully into the recored
side decks.
 wrote:
 I am considering a purchase of a long neglected sail boat. I am
 confident of my ability to assess and estimate time & costs for mot of
 the issues involved in bring this vessel up to good condition, except
 one problem. The cored deck is soft, very soft in some places. The cabin
 top is not cored and is in good condition, except around a couple
 opening ports where the ports were torqued down so tight to stop leaks
 that it began cracking the fiberglass.
 But it's the decks that worry me as I have never done a repair of balsa
 cored decks, nor had reason to consider such costs previously. This boat
 is out of the country, btw, so I would be depending on yard work done in
 either Grenada, Carriacou, or Trinidad. Before talking with yards about
 this, I would like to have better info of what is likely to be involved
 so that I can better assess their plans, proposals, costs. This is for a
 36' boat.  Recommendations on yards or personnel in those islands
 welcomed.
 I can feel the softness, and in many places see the indication of cracks
 in the nonskid around the soft areas, pretty much defining the places I
 feel. In one place I can make the deck squeek by standing with feet
 apart and just shifting weight between them.
 Can anyone relate their experiences and costs with such repairs? Or
 point me towards good source materials about the subject.
 I am also wondering whether I could sail the boat between the islands
 mentioned above to get to a preferred yard for the work. The mast is
 deck stepped, the cabin top under the mast seems stiff, and the shrouds
 are not deck mounted but rather to the hull and internal bulkheads. I
 could also motor as the engine seems fine for its age.
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at:  http://www.rutuonline.com
Shameless Commercial Division: 
http://www.spade-anchor-us.com