my decks are not just wet they are actually missing in some places. Extensive water intrusion and also bulkhead damage... more Spring jobs :-|
The core in the area of the new stanchion will be the same as that in the existing stanchions, the installation of which was not done properly. If your core is good where you are mounting the new stanchion you should overdrill through the top layer, excavate core in the area 3/8 or 1/2" back into the deck, fill with hardened epoxy then drill for the new stanchion bolts. This should also be done for all existing stanchions (and all existing deck hardware... something the builders just not do right). The outboard holes for the stanchions may be through solid fibreglass but the inboard holes will be through cored areas. While you are doing it, it would be a good form to 'paint' the insides of the outboard holes with unthickened epoxy resin (maybe twice) to reduce water permeation into the fibreglass laminate (redrill slightly oversized first).