If I understand your question correctly.
When originally built they put resin putty on the top of the stringer to adhere to the bottom of the deck and inner liner (all one piece) to create a solid piece. That gap is approx 1” at least on my boat. Over time that putty broke and cracked and did not adhere very well to the deck and also could cause pressure dings on the top of the foam filled stringers.
The method of redoing the deck determines what you should do.
If you are Not going to raise the deck up 2-4” but keep it as is you could just remove the top layer of glass remove the core material and replace with new core material and replace the top layer of glass.
If you are going to raise the deck up then you should keep a 2” rim all around the outer deck for support plus add additional lamination to the rim and also to the stringers to mach that height. Then laminate your deck on both sides with glass adhere it to the stringers and deck rim glass additional layers to fuse it into the inner liner and finish it off.
On my 23’ I did not raise the deck, on most 20” and smaller they raise the deck. If you have a 23’ and are going with twin configuration I would raise the deck to keep the scuppers above the water line.
FellowShip
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Just for the Grins
Official 23’ SF Antique Classic SeaCraft Owner