a deck. Take a 3 inch #12 wood screw and cut off the head with a hacksaw. Now bend the screw 90 degrees about 3/4 of an inch from the tip end. Once you have the bend made chuck it up into a 1/4 inch drill. Use a hole saw and drill/cut down through the top fiberglass and the wood core but not down through the bottom fiberglass. You will have a 1/4 inch hole all the way through but you cover that small hole with a piece of masking tape. Once you pry out the center of the deck stick the bent screw into the blind hole and start digging out the core between the top and bottom fiberglass deck and it's underside. The drill will want to bang around a bit but just keep digging away and it will do it's job. Do this all around any shape you cut out and then fill the cavity with micro bloon reinforced with epoxy and fiberglass. You end up with a sealed edge that is 3/4 of an inch back under the deck. I use this method anytime I cut into a cored deck for any reason and it works especially well for mounting deck hardware that is through bolted.