Water might be the environment that boats operate in, but water can be the most destructive ingredient when it comes in contact with foam or balsa deck and hull cores. Leaking water can react to the resins and adhesives in fibreglass core material and cause a chemical reaction that melts and deforms the core. Serious fibreglass delamination results. The reason for the destruction commonly points to the inefficient technique used to attach the hardware to the fibreglass boat. Any boat owner can follow some easy steps and use a few special products to protect his boat against thousands of dollars in damage.
Mix dish washing soap in a bucket of water. Use a cleaning brush to scrub the area where the hardware fitting will mount. Remove all oil, grease and wax. Dry the area with clean rags. Set your piece of hardware over the surface where you want it mounted. Align it properly. Mark an outline of the hardware piece with a felt pen on the deck or fibreglass surface.
Use a scratch awl to stick down into the mounting holes of the hardware to mark the drill holes in the deck or surface. If hardware needs to be mounted above deck, look under deck for the proper clearance needed to attach the nuts and backing plate. Apply masking tape all around the perimeter of the hardware piece you outlined with the felt pen. Extend the tape out a few inches in all directions.
Take a piece of aluminium backing plate, which measures 1/8-inch thick (for a small-stressed hardware piece) and cut a section out with a hacksaw, large enough that it will accept the drill holes of your hardware piece. Size the backing plate under the deck (or underside of the top surface), to make sure it will fit and lay flat. Place your hardware piece over the backing plate and mark the drill holes with the scratch awl.
Use the drill motor and appropriate bit to drill the holes in the backing plate that match the hardware piece. Open a can of polysulfide adhesive caulk and dab a liberal layer over the top of your marked area with a small plastic spatula. Put a coating of adhesive caulk on the underside of the hardware piece.
Use the spatula to apply a layer of caulking adhesive on one side of the backing plate. Instruct an assistant to take the backing plate below deck or hold it to the opposite contact side of the mounting area.
Instruct the assistant to align the backing plate with the drill holes, placing the caulked side face up against the underside of the deck. Tell him he should put on one spacer, one lock washing and one nut (in that order) onto the bolt threads when they appear on his side.
Place the hardware piece in position over the drill holes. Coat the bolt threads (or screw threads) with caulking adhesive. Push the bolts down through the hardware piece. Have your assistant secure the spacers, lock washers and nuts on the ends of the threads. Have your assistant tighten the nuts by hand and them hold the nuts steady with an end wrench. Use a socket and wrench to tighten the tops of the bolts, but screw them in only snug-tight with medium pressure.
Scrape the excess adhesive caulk away from the seam underneath the hardware piece with the spatula. Remove the masking tape. Wait one week for the adhesive caulk to dry and cure. Then re-tighten the hardware piece bolts with firm pressure. This will seat the hardware piece into the adhesive caulk, which has now formed a gasket.
You can use 3/8-inch marine plywood as a backing plate for small stressed hardware pieces. For stanchions and heavy cleats on a large power or sailboat, use 3/4-inch marine plywood as a backing plate. If you want to use aluminium for very heavy hardware stress points, obtain aluminium backing plate that measures 3/8-inch. Always use through-bolts and backing plates to attach any piece of hardware to a fibreglass boat. Screws, even if they come with the hardware piece, will tear out of the fibreglass material when subjected to even the lightest stress. Use extra-large washers to distribute the torque stress. Use some acetone and a rag to remove any traces of adhesive caulk that might have stained your deck.