Capt'n Pauley's Pages

Sealing Decks

by Paul Esterle

We all like to customize our boats, adding a handhold here or a drink holder there. In some cases, we may need to replace a fitting or add a substantial cleat. The average boat owner can easily do all these things. However, there are a few things you need to know before you start drilling holes in your boat.

Before I review the proper procedure for making those holes in the boat, let’s talk about how boats, and primarily decks, are made. Almost every production boat deck is made in a female mold. The first step is to thoroughly wax the mold. This allows easy removal of the deck from the mold and provides the smooth, shiny surface you will see in the finished deck.

Next a layer of gelcoat is sprayed into the mold. This coating provides the color and weather resistance expected from a fiberglass surface. A fiberglass mat material is then applied against the gelcoat. This mat is placed next to the gel coat to keep the pattern of the serious reinforcements; fiberglass cloth, woven roving or exotic materials such as Kevlar or carbon fiber, from showing through the gelcoat. These reinforcing layers provide most of the strength of the deck unit.

The reason for building decks this way is one of weight. A cored deck is stronger, stiffer and lighter than the equivalent thickness of fiberglass. That’s the good news. The bad news is that water leaking into that core can seriously weaken the deck structure. The result is decks that are springy (soft) and delaminated, resulting in serious structural problems.

The purpose of the remainder of this article is to give you some ideas on properly sealing those holes you already have in your deck or others you need to drill for new additions. In future articles I’ll cover some of those projects, but this article will serve as a primer.

Uncored Decks
You’ll discover if you have a cored deck when you drill that first hole through it. If you get nothing but fiberglass chips, you’re home free. In this case all you need to do is to apply a coat of epoxy to the sides of the hole. If it is an existing hole, clean up the sides with a Q-tip and a solvent like alcohol or acetone. This will clean off any grease or dirt before the epoxy is applied. Once the epoxy is set, you can proceed with bedding or re-bedding your hardware. I’ll cover that process in another article.

You may wonder why the fiberglass needs sealed. The strands of glass in the fiberglass will actually wick up water through the ends of the strands. This water can eventually help develop blisters. It is admittedly a little farfetched but why take chances?

Cored Decks
Okay, so you drilled your hole and found wood or plastic foam chips, indicating a core, what now? Well, the process is slightly more complicated. You will need to clear out the core around the hole (about 1/2” to 3/4”) and apply an epoxy/filler. This epoxy/filler mixture does two things. First of all, it seals the core material from any water contact. Secondly, it provides a strong “bushing” or pad that won’t crush the core as you torque down the fastener.

There are several ways of cleaning out the core inside the fastener hole. Once is to take a small Allen key and cut the short end off to form a cutter (see Drawing #2). Chuck it up in your electric drill, stick it into the hole and slowly start up the drill. You can also use a variety of bits in a rotary tool. You need be able to chew up the core and vacuum it out. When the hole is cleaned out, seal off the bottom with tape or a small piece of children’s modeling clay. Use the white colored clay to avoid any color bleeding onto the fiberglass surfaces. Make sure the tape or clay seals completely; you don’t want any chance of epoxy dripping below. Give the inside of the hole a coat of unthickened epoxy to seal the core material and then fill the hole with a thickened epoxy mixture. Use high-density filler, such colloidal silica, that won’t compress when the fastener is installed.

After the epoxy has cured, you have sealed off the core material from any chance of water leaks and provided a strong point for fastening your hardware in place (see Drawing #3). You can proceed to re-drill the hole and install the new piece of hardware or reinstall the old. This same idea and technique can be used on any opening through your deck. The openings for widows, hatches and portholes are often cut out and the edges (of the opening) are left unsealed. This process assumes that the sealant will never leak or need replaced. Far better to remove the port, seal the edges of the opening and then properly re-bed the port.

Paul Esterle (www.captnpauley.com) is a freelance boating writer and “man for all vessels” with many years and a wide range of boat maintenance experience. He publishes in popular magazines such as SAIL, Blue Water Sailing, and Good Old Boat, and is Technical Editor for Small Craft Advisor Magazine. In addition, Paul has produced a series of boating related videos and CDs, including the instructional video used by Spartite(TM) at boat shows. Paul has also hosted seminars and lectures at various boat shows and marine stores.

Paul can be contacted directly at pesterle@preferred.com with any questions or suggestions.

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