According to John Isaksen, Neptune (35), and confirmed by Tommie Chen, the teak decks are held down with a black adhesive. It's the same polysulfide-like material used in the deck seams and window jams. The decks' teak was originally 1/2" thick. This was determined by removing the fuel-deck-fill-plate, which protected the underlying wood from erosion, and measuring its thickness. The deck's overall thickness at the filler port is 1-3/4". The fiberglass appears to be 1" thick. There's a 1/4" of plywood under that. The filler port appears to have had a 3" extension spout welded to it.