To install a cleat, for example, you would first drill the holes to match the diameter of the fasteners. Then a boatyard professional would use a router with the proper bit to remove the core material from between the inner and outer FRP skins.
Do-it-yourselfers can use a pick or better still, a file point heated and bent over 90°. Remove or reef out the core for a distance of at least half the hole diameter, no less than a quarter of an inch (a half-inch fastener hole would get a quarter inch of core removed from the perimeter of the hole, but more is OK). Cover the bottom of the hole with masking tape, and then fill the hole with thickened epoxy (the consistency of mayonnaise).
Once the epoxy dries, redrill the hole, remove the tape (it’s okay if some of it sticks to the epoxy), bed and install the cleat/fastener using quality marine sealant and a backing plate made of aluminum, stainless steel or substantial phenolic laminate. Washers are not adequate backing plates for high-load components like cleats, windlasses or winches. When the sealant fails, the water will leak into the boat but not the core. Some pros opt not to bed the inner part of the fastener or backing plate, so a leak will make itself known rather than filling up the hole and trying to find its way through the epoxy annulus.
The procedure is applicable to any hardware installation, from snaps to windlasses (not only the fasteners, but where the chain passes through the deck as well), cleats and ports. Self-tapping screws like canvas snaps need not penetrate both skins but still require overdrilling and epoxy backfilling and redrilling. Ports require reefing the core back at least as far as the fasteners securing the trim rings, which could be as much as 1 to 2 inches. Once again (it’s worth repeating), no hole should be drilled into a cored laminate, regardless of how small, without following this procedure.
The benefit of the epoxy insert is that, in addition to sealing the core from possible water intrusion, it also prevents the core from being crushed when the fastener is torqued. Make no mistake about it, torquing even lightly loaded fasteners will compress and crush the core material (you’ve probably seen those indented areas around deck hardware installations), leading to a weakened structure, leaks and core saturation.