Deck structures with a foam or balsa core present issues not faced with a solid fiberglass laminate structure. These core materials have low compression strength and the torque required for properly fastening deck hardware (not to mention the loads seen with use) will crush them. It is necessary to supplement the core at the point of attachment of the hardware to prevent this. The process described here has been used for many years successfully on many core types. As a side note, if your boat has a plywood core, this work is not necessary as its compression strength is quite high.
Materials/ Tools required:
16d nail or similar
Filler for resin
You will begin by creating a tool for clearing the core out of a fastener hole. Cut the head off a nail and in a vise or between two pair of pliers, bend the pointed end over at 90 degrees. The length of the bent leg should be equal to or slightly longer than the diameter of the fastener to be used to mount your hardware.
Locate the hardware in its desired location and mark the fastener holes . Drill thru the deck at each location using the proper bit for the fastener size.
Fit your bent nail into your drill motor and insert the tool into each hole in turn and slowly run the drill motor. This will spin the tool and gouge out the core material in a cylinder shape between the fiberglass laminates.
Vacuum the loose material out of each hole and place a piece of tape on the backside of each hole. If you are working in a nonskid area it is recommended to cover the surrounding surface with masking tape and cut open each fastener hole. It will make clean-up much easier.
Mix your filler to a consistency somewhere between peanut butter (smooth style) and mayonnaise. Thicker is better but is more difficult to get into the holes than a thinner mix. Work the filler into each hole by sweeping the putty knife back and forth over the hole.
When the filler ‘burps’ up at the hole margin, it is likely that all the air has been expressed out. This step does take some time to accomplish and it is important that the air be displaced.
When all the holes are filled, trowel off any excess filler from the surface, pull the masking tape and remove any spilled filler. Allow the filler to cure.
Now re-drill your fastener holes for the hardware and install The accompanying photo series shows a cross section of the work.