Fiber glassing question

The simplest way if you are going to use FG it to abrade the area where the grab bar will attach, solvent wipe with acetone, and lay 3 to 4 layers of woven glass over the flange of grab bar over lapping 3-4 ins on the deck itself. An 8 harness satin weave is easier to form around the bar than a square weave. I would use an good epoxy resin with the FG cloth maintaining a 60-40% cloth to resin ratio. If you go with a wood board and an adhesive like 5200 the adhesive wont fail but the wood bond line may if you bang it or put enough load on it. If you bond the board flat to the floor the wood fibers are very weak in that direction. Think how easy it is to split wood in that direction. Cab o sil is good for thickening the resin and works well for making a filet but adds nothing for strength. I would not be afraid of through bolts leaking if sealed with the 5200 or 4200 and would be the strongest solution. I am guessing your hull is at least 1/4 in thick.