Coring in 7 easy steps

Most fiberglass boats have some type of core material like wood or foam which helps add strength while keeping the weight down.   

Privilege catamarans are fiberglass with divinycell foam core.  When screws and bolt holes are made in the fiberglass to mount something, the something is caulked to keep water from getting into the core.   

Ultimately all caulk leaks and water will, and has, infiltrated into the core.  Once it is in there, it usually stays localized but may stay there forever.   Worse yet, with a wood core, the wood will rot.

Since we removed every bolt and screw for the spraying, it was a good time to take care of these problematic holes.

A couple of things that are not shown which are nice to do are leave the hole open to the baking sun for as long as possible to remove any old moisture and when reinstalling the screw, I preheated it in boiling water to help it slightly melt the epoxy plug instead of cracking it.

Here is the coring process I used:

Originally installed snap

Snap removed, notice superficial gelcoat stress cracks

Drill with a step drill to avoid chipping the gelcoat

Drill out the core.  Do not drill through inner fiberglass

Use a bent nail in a drill to remove core material

Tape the hole and use the step drill to make a nice clean hole in the tape

Spray out all the dust and old core particles

Inject Epoxy throughout the hole

Solid core plug.

Drill a hole for the screw

Completely watertight.   Finished