Potting & Countersinking

[cas8100]

I have two questions about rebedding parts on the deck and was hoping those with more experience than I wouldn't mind chiming in here. There's a ton of threads on this topic, but an understanding of the terminology and basic steps are often assumed.

Potting a hole - is this basically pouring unthickened resin in the hole to seal it from any type of possible water penetration? If yes, do you generally drill the hole larger to allow for the thickness of the cured epoxy?

Countersinking - a process that seems to be highly recommended. From what I understand, it's simply taking a countersink bit and grinding away a very slight bevel around the surface of the hole. Do you do this even if there's a plate between the bolt head and the fibrglass (i.e. Chainplate)?

Thanks all.

Carl

[arf145]

MaineSail has an excellent, detailed post on this (sorry, don't know link).

Actually, potting is filling a slightly enlarged hole with thickened epoxy to seal the core from water penetration. Some of the core should be removed beyond the hole diameter to better seal the core and to give the epoxy a larger area to grip to.

Yes, you countersink even with a plate. The countersink isn't for a screw head to go into, it is to provide a ring around the hole that will contain sealant and act as a gasket. So you should put bevels around the screw holes beneath any hardware, such as chain plates, deck organizers, etc.

[Freesail99]

Carl, you have the right idea, good luck.

[sailingdog]

Originally Posted by cas8100

I have two questions about rebedding parts on the deck and was hoping those with more experience than I wouldn't mind chiming in here. There's a ton of threads on this topic, but an understanding of the terminology and basic steps are often assumed.

Potting a hole - is this basically pouring unthickened resin in the hole to seal it from any type of possible water penetration? If yes, do you generally drill the hole larger to allow for the thickness of the cured epoxy?

No, potting a hole is a bit more than that. It is removing the core from the area around the fastener hole, and then coating the void with unthickened epoxy, to coat the fiberglass and core material, and then filling the void with thickened epoxy.

The two reasons for doing this are simple. First, you want to protect the core from water intrusion, which coating it and filling the void with thickened epoxy will do. Second, it will help strengthen the laminate against the compressive forces created when you tighten the bolts down.

Most core materials are not all that high in compressive strength—thickened epoxy has a pretty high compressive strength...so it makes much more sense to have thickened epoxy there to support the loads.

Countersinking - a process that seems to be highly recommended. From what I understand, it's simply taking a countersink bit and grinding away a very slight bevel around the surface of the hole. Do you do this even if there's a plate between the bolt head and the fibrglass (i.e. Chainplate)?

Thanks all.

Carl

Yes, since the void created by the countersinking will allow the sealant to form a natural "o-ring" and help create a long lasting, flexible and durable seal.

BTW, here is Maine Sail's website page on sealing the core.

[juda1]

Originally Posted by arf145

MaineSail has an excellent, detailed post on this (sorry, don't know link).

Actually, potting is filling a slightly enlarged hole with thickened epoxy to seal the core from water penetration. Some of the core should be removed beyond the hole diameter to better seal the core and to give the epoxy a larger area to grip to.

Yes, you countersink even with a plate. The countersink isn't for a screw head to go into, it is to provide a ring around the hole that will contain sealant and act as a gasket. So you should put bevels around the screw holes beneath any hardware, such as chain plates, deck organizers, etc.

I would've posted this up but you got it before me

[Maine Sail]

Originally Posted by cas8100

Do you do this even if there's a plate between the bolt head and the fibrglass (i.e. Chainplate)?

Yes because the seal is not the head of the bolt but rather the shank of the bolt and the underside of the plate. By creating a chamfer/bevel it forces sealant around the bolts shank which prevents moisture intrusion. Sealing the head between the plate and bolt head adds minimal protection though you can still roll an o-ring of butyl to wrap under the head if you'd like or use 3M 101.



Look closely at how the sealant is forced into the threads and against the chamfer/deck on the right and the non-chamfer one or the left has no seal against the bolts shank/threads.



If you really want a great seal chamfer the stainless deck fitting ever so slightly too on the side that faces the bolt head..

[cas8100]

Thanks for the replies. Any suggestions on a tightening process for rebedding parts? I've seen some people hinting about a "2 step tightening process"

[Maine Sail]

With countersinking you tighten once! Try not to move the bolt and only the nut. A slight twist is fine but multiple rotations with butyl is not so good..

[jbondy]

Maine Sail,

I've been seeing the recent references to sealing using butyl tape. Your photos don't show this layer. When using butyl tape, is there a sealant layer under the tape or does the use of butyl tape obviate the need for any other sealant?

TIA

[Maine Sail]

Butyl is the only thing used in that article. Look close and you'll see it squised in there and around the threads.

This is butyl: