Filling a six inch deck hole to 3 inches

[antongm]

I'm recycling a huge six-inch hole in my cabintop from a since-removed diesel stove flue. The idea is to put in a Nicro solar vent, that requires a 3-1/4 inch hole (please see attached photo).

What's the best way to do this? Should I just tape up everything from the interior, fill it with thickened epoxy, and drill out the right sized hole? Should I try something like a mold instead? Something different altogether?

FYI: there's plywood coring to the deck, plus about an inch of plywood and cedar planking beneath the fiberglass as a sort of wooden headliner.

Any ideas appreciated.

[40 South]

Cut a 6" plug from an epoxy glue laminated piece of ply with a hole saw. The type that has a pilot drill with a cup shaped circular saw cutter. Then cut a 3 1/4" inch hole from the 6" disc. Then epoxy glue the resultant "ring" into the hole in the deck. Finish deck and inside to suit.

[roverhi]

The right way to do it is to bevel the edge of the hole in the deck and laminate up 2-3 layers of matt and cloth. Fill the hole with a piece of wood to match the interior. You can cut the plug with a saber saw or bandsaw and sand the edges till it's round. I made up a board to set a belt sander on and used that to finish sand the piece round for a similar repair. Another way to do it would be to glue a piece of teak to the deck slightly larger than the existing hole and drill that for your vent. Do the interior repair the same as above

[kentobin]

More reference material from West System:
Fiberglass Boat Repair and Restoration
WEST SYSTEM | Projects | Fiberglass Boat Repair & Restoration - Repairing machined holes in fiberglass

[Azul]

Just fill in the six inch hole with a piece of marine plywood, West system epoxied in. You will need to use an orbital sander and grind out a tapered "divot" twice that width in the fiberglass and fill it with at least two layers of target shaped torn pieces of chopped strand mat. Then several applications of polyester fairing compound like Evercoat. Forget the Nicro, I took out two of those worthless pieces of garbage. Noisy, leaked, and very little air flow. If the hole is in your deck, it will show somewhat no matter what you do if there is intact non skid surrounding it.

[Azul]

Maybe I should mention you can tape over the bottom of the hole with white duck tape, epoxy doesn't stick to it. The wood plug doesn't need to be a perfect fit, it won't go anywhere if you use epoxy thickened with structural filler. I had those solar Nicro units installed by the PO... expensive, quit spinning immediately with any shading after the batteries failed... and they leaked into my deck core. Ugh.

You can read about how to do the CSM fiberglass repair at http://boatworkstoday.com/video-page