Potting deck hardware, keeping the core dry

I need help, and Im hoping someone here could give me advice. I want to drill the holes oversized fill with a thickened resin and re-drill for the fasteners. The problem is my boat has a top layer of glass, about 1/4" thick and then a plywood "core". There is no fiberglass under the core. to make matters worse the access points to any fasten is through the cabin fiberglass liner. The access points are only large enough to get a socket through to tighten the nuts on each fastener. Typically I think I would drill the hole and then tape the bottom side to keep the thickened resin from falling out. Without access to the underside of the core I have no way to keep the thickened resin from falling out. Any ideas on how to accomplish this?

 

I don't think there is much need to pot fasteners that pass through a plywood insert,the technique is usually used to prevent crushing a foam or balsa core.You do need to make every effort to prevent water intrusion and you can do this by painting varnish or resin around the inside surfaces of the holes.they may need re-drilling once the sealing liquid is dry.You will have the problem of getting a good size washer in contact with the inner surface in any event and a really highly loaded fitting might require a backing plate.Given this need for access,it might be a better long term solution to accept the need for an access hatch in the headliner.You could cut a neat hole and edge it with a rebated wooden section that the loose piece fits into.It would look less bad than a series of 3/4" holes,but if the hardware is a four foot long handrail you will have no way of avoiding a line of holes.

 

How about a premade fiberglass tube with proper inner diameter glued into a snug-fitting hole so that the epoxy is more of an adhesive and sealer than a bulk filler material.

PS added pic after a couple replies

 

Took me a moment to understand what you were saying but I finally understand. That’s not a bad idea. I’ll think it over.

 

Wet Feet pretty much nails it. For an extra layer of certainty; you can also paint the inside of the cutaway with epoxy or epoxy slurried with cabosil.

 

Plenty of ideas, from cut to size cocktail umbrellas to more fiddly stuff. But the easy option is also very simple: thicken some epoxy until it's stiff and smear a small amount in the hole from the inside plugging it. Wait until it kicks and fill your hole from above as usual. Epoxy repair putty is the ready made option for this.

 

You don't give the fastener size, but I'll use 1/4in. as an example.
What you don't want to do is drill 1/4in. holes and then have to beat the bolts thru them, that just removes the caulk from the bolts and makes any future removal more difficult.
Drill your hole pattern as good as you can, but making the holes ~1/64>1/32 oversize with a little countersink on the outside.
If you wish you can swab some epoxy in the holes.
Then you can use your caulk on the fitting/bolts and put things together.
The little countersink area will allow the caulk to form a "donut" around the fastener that won't get squeezed out, and the slightly oversize holes allow for caulk and reduce the amount of colorful language needed for removal.
Absolutely nothing wrong with internal cover plates to hide nuts/bolts, done well they show attention to detail.

 

I don't think I would be drilling oversize holes for any fitting that has a shear load.I don't recall ever having a bolt fail to slide through a hole that was drilled the nominal size of the fastener and I always regard it as good practice to apply some mastic to the top portion of any fastener.After all,a thread is a wonderful helical leak path if nothing is applied to prevent water getting through the gap it includes.I have had sales people pushing bolts with rolled threads and plain shanks that were the diameter of the thread root.While this may be a good thing in terms of stress,it does nothing for lateral location or prevention of water ingress.Such things work better on castings that have ring dowels inserted on both pieces with the bolt passing through the middle.

A small countersink is sound practice and anybody who has looked closely at non-countersunk bolts will have noticed that they have a small radius where the shank meets the head,so another instance for introducing a little clearance.

 

In a lifetime of boat work I've come across far too many instances of hardware that used multiple fasteners, (like cleats/winches,) where just a tiny bit of misalignment in the holes has led to the fasteners being beaten in with a hammer.
Caulk needs room to work, and a 1/64th oversize in a hole is of no concern on shear loading of any boat hardware which is being fastened to wood/fiberglass.
Having the caulk extend down into the mounting surface a fair amount is good practice.
And cranking-up the fastener torque is not a way to prevent or stop leaks.
Too many installers of boat hardware use wrenches as though they're tightening the head bolts on a diesel engine.