Sealing Deck Penetrations To Prevent Core Rot

The Epoxy Supplies

Most all boats have some sort of “core” in the decks. Whether it’s balsa, plywood or some of the foam cores all can eventually absorb water from a leaking deck penetration. A leaking deck, especially with wood cores, can lead to core rot and eventually delamination of the deck and loss of structural rigidity.

Sailboats especially can have hundreds of holes drilled into the deck for cleats, stanchions, blocks, winches, genoa tracks or many other mechanically fastened and through bolted or screwed in items.

Unfortunately most builders do not take the time, nor have the patience, to protect the core from moisture. Most builders also fall short in their bedding techniques, choosing in favor of cost cutting measure that you get to deal with a few years beyond the warranty. The “can’t be seen, must not be there” mentality certainly rings true in the category of deck hardware installations.

The re-bedding of deck hardware is a time consuming task but one that can save your five, six or seven figure vessel from loosing 25% to 50% of it’s market value due to saturated and rotted decks.


 There is no method better than potting the holes with thickened epoxy to prevent core damage, if you have core where hardware passes through it. This project  is easy but also quite time consuming. Once done correctly you will never have to worry about water getting into your core again.

These are the epoxy products I often use to pot deck holes. I sometimes use System Three Epoxy products too but for this article I used West System. West System epoxy products are very easy to find and use. I use a #205 hardener #105 resin & Colloidal Silica or “Cabosil” as the filler or thickening agent.

In order to inject the resin into the holes I use the West System syringes.

Brad Point Drill

When drilling into fiberglass the proper tool is a must. This is a brad point drill and it will create a clean, no chip hole in gelcoat provided you use a few tricks.

TIP: The main trick with brad points into gelcoat is to start the drill in reverse. This trick also works well with hole-saws. By starting in reverse it allows the outer cutting edges of the drill to cut cleanly through the gelcoat. Once through the gelcoat you can then switch the drill to forward and complete the hole without chipping any gelcoat off the surface.

Dremel Bit #115

This is one of two bits I use to remove the core. It is a 5/16″ diameter head with a 1/8″ shank. It allows a just right amount of core removal to seal the deck from moisture.

Enlarging An Existing Hole

In order to complete this article I used a scrap piece of end grain balsa deck structure. I wanted to use a scrap piece so that I could make cut-a-ways to show the inner workings and what actually happens on the inside of the deck.

Here I’ve drilled three holes;

• One hole for the #115 bit,
• One hole for the #654
• One hole that will remain as 1/4″ with no epoxy fill.

The center hole, as you can see, has been counter sunk. This was originally a 1/4″ hole that I over sized to 5/16″ to make the #115 bit fit into it. Over sizing holes in fiberglass can often result in an “oops” and some serious deck chipping.

TIP: Here’s the trick to over sizing holes:

#1 Countersink/chamfer the hole to just barely beyond the width of the drill you will use for the over sizing, in this case 5/16″. If your countersink bit is new and sharp get in the habit of using reverse, not forward, when countersinking fiberglass. Forward will remove too much material, far too fast, if you are not careful.

#2 Place your brad point drill in the chamfered hole and start in reverse. You will cut through the top skin only and have a nice clean hole. You do however need a brad point drill set. You won’t find them at Home Depot and will find them a Rockler or on line. This is the quickest and cleanest way to bore the top skin without the drill wandering or causing serious gelcoat chipping.

Drill For Making Over Sized Hole

Here’s my 5/16″ bit that I used to enlarge the hole on the top skin. Again, do not drill all the way through the deck just drill through the top skin.

Bottom Holes

This photo illustrates that all the holes below deck are still 1/4″ even though the middle hole has a 5/16″ diameter on the decks surface. This leaves as much of the deck skins intact is as humanly possible.

Inject Non-Thickened Epoxy First

I know I said “thickened epoxy“, and we’ll get there, but first it’s a good idea to inject clear mixed non-thickened epoxy into each hole. By doing this you allow the less viscous, resin only, without thickener, to penetrate the surface pores of the core. this creates a better and stronger final bond.

In order to do this I inject straight mixed epoxy into each hole with a syringe. I then let it sit in each hole for about 3-4 minutes, suck it back out with the syringe and then squirt it back into the mixing cup where I add Cabosil (colloidal silica) to thicken it.

Remove Trapped Air Bubbles

Here I have re-injected the holes with my West System syringe. I did cut about .5 cm off the tip of the syringe which makes the hole bigger to accommodate the more viscous thickened epoxy.

TIP: The green stick you see is simply a piece of weed-whacker line. This is a very simple and cheap tool for popping any trapped air bubbles and aiding in migrating bubbles to the surface so you leave behind any voids.

All you do is move the weed-whacker line in, out and around the hole until you have no more air bubbles rising to the surface.

Wipe It Down

Of course if I were doing this on an actual boat I would have first pre-taped over the holes, like the photo above, & then used my countersink bit between my fingers to remove the tape from the holes. This method protects the deck and the countersink carves out a nice clean hole in the tape.

In this photo I simply used a rag with Acetone to wipe away any over flow. Use tape and it will go much smoother and cleaner. I simply forgot when photographing this..

#654 Bit – Cut-A-Way of Potted Deck Hole

After I removed the tape I re-drilled each of the holes with my 1/4″ drill and then counter sunk the two holes that had been potted.

This is a picture of the hole I made with the 1/4″ #654 Dremel bit. The hole is clean and precise with plenty of separation between the epoxy and the core. Both the top and bottom holes through the deck skins remained at 1/4″ for a factory original look and feel yet this is much better than how it would have come from the factory.

Explanation of Countersinking

“RC why on Earth do you countersink or chamfer the deck?”

#1 It does a lot to prevent gelcoat crazing. By feathering the edge of the gelcoat, through the use of countersink bit, you are less likely to start a crack or craze mark in the gelcoat.

#2 By creating a bevel or countersunk recess in the surface of the deck the marine sealant or Bed-It Tape has a cavity to fill. Without a bevel the hardware would compress the sealant to about 1/64th of an inch thick after the fasteners are tightened. Marine sealants do have some flexibility but not as much as you would guess.

Follow me for a moment:

Let’s say you have a marine sealant that has a rating of 400% elongation before break on a 1/64″ thick joint. Doing the math shows us that 400% of 1/64″ is only 1/16″ of total allowable joint movement before a joint failure or leak starts. In the case of a stanchion base 1/16″ is not much allowable movement before a sealant failure.

By countersinking around the bolt holes you’ll increase the maximum thickness of the sealant, at the bevel, to roughly a 3/32″ depth at it’s deepest point. Using the same math as above 3/32″ X 400% gives you a total joint movement before failure of 3/8″. If a mechanically fastened joint is moving 3/8″ you have more problems than just a deck leak. So, 1/16″ allowable movement before failure at 400% without countersinking or 3/8″ allowable movement at 400% before failure with countersinking.

Even a small bevel will drastically increase max allowable movement before a sealant failure. Countersinking to a mere 1/16″ depth will give you 1/4″ total movement around the bolt before failure. You don’t need a deep bevel to make a large difference between a failure and a seal.

#3 Countersinking is really a very simple step and has numerous benefits. For instance, you can install the fitting and tighten it down now with no waiting. This means you can seal deck hardware alone and not wait for a marine sealant to change states or cure.

I am going to inject some strong opinion here so please move on to the next photo if you don’t want to hear it. I would discourage anyone from using the Don Casey “two step“, “wait to tighten and form a gasket” method of bedding deck hardware with a curing marine sealant.

Is this because it can’t or won’t work? Absolutely not? Is it because I don’t like Don Casey? Absolutely not! Don is a great guy and tremendous asset to the boating community. Then why?

In my experience this method is probably one of the leading causes of deck core rot on the planet. This is NOT to say that when done properly it won’t work, just that it is VERY, VERY, VERY, VERY difficult to actually do this properly. Many a DIY and pro have messed this up and ruined decks as a result.

Think about it:

If the sealant cures, to form a gasket, and you then move the bolt while tightening it down on the second step…. You will likely lose. If you create a gasket too thick, it cures, and then you can’t get the mechanically fastened hardware tight enough, against the deck, because of compression resistance, the hardware will move, flex and also fail, so again, you lost the battle.

I mean no disrespect to Mr. Casey, he writes tremendous marine books and articles, but the method he made popular is very difficult to nail down repeatably because cure-rates change with humidity, temp and far too many factors to get a good reliable procedure you can easily repeat.. I have witnessed far too many cases of core rot due to DIY & professionals using a two-step bedding procedure than I would have liked to.

So where did I lean this countersinking method? I learned it from Hinckley Yachts… Sure this extra step takes all of about 30 seconds per stanchion but apparently it’s too much extra work for most production builders.

Not Much Stretch

This bolt hole has no bevel at the deck level. It was loosened and raised the same amount as the previous bolt, to illustrate how much difference in allowable movement one can attain through the simple step of countersinking.

Alternative Methods

This photo represents two other ways of doing the same job. Personally I’m not a huge fan of either one as much as the previously mentioned methods using the Dremel bits.

Why don’t I like these methods as much?

#1 In method #1 I drilled an over sized 1/2″ hole clean through the deck and then filled it with thickened epoxy. By over drilling the entire thickness you have removed the deck skins which are the major structural component in the I-beam. End grain balsa has very little strength along the grain, which is where the epoxy adheres to in an over drill. Picture splitting fire wood. You split with the grain and if flakes off nicely. Epoxy adhered along the grain can do the same.

If you tried to split across the grain or pull balsa apart via the end grain it would be very, very difficult and this is why it is called end grain balsa and why the deck skins are laminated to the end grain not parallel to the grain of the wood.

The areas highlighted in red, the deck skins, are thinner than they appear in this cropped photo, especially on light weight sailboats. In this example your only means of structural support are the circumferential bond between the top and bottom skins as the area in green is very, very weak comparatively speaking and would really rely on the backing plate.

I have seen two failures of total over drills in my life. Theoretically speaking, when used with proper backing plates it should hold but if your balsa is even just moist your entire bond will only be the thin area of the two skins. It should be noted that epoxy does not adhere well to wet core this is why it is critical to seal the deck before it gets wet.

#2 In method #2 I have only over drilled the top skin but left the bottom skin intact. This is stronger than method #1. With the top skin gone it too is less strong than leaving most or all of the top and bottom skins intact and creating a captured plug.

#3 Using either method #1 or #2 leaves you with a larger hole on the UV side of the deck. Some stanchion bases or hardware may not entirely cover the larger hole completely and epoxy does not like UV. This means an additional step of gel coating the repair.

It’s entirely up to you how you seal your decks all I can do is present the pro’s and cons, as I see them. Of these two methods I prefer method #2 but I still prefer the Dremel procedure far and above both methods #1 & #2.

That said ANY of these methods is far superior to not sealing the core off from potential leaks.