Strategies for Sealing Deck Hardware

 Think you don’t need a “strategy” for sealing decks? Don’t think your deck is even leaking? Well, you may be right, but your deck may well be leaking below your feet. Deck leaks can be insidious, going on for years before manifesting themselves with drips from the overhead and damp interiors.

  The Importance of Sealing

To understand why sealing a deck properly is important; we first need to understand how most decks are made.

 

Almost any deck built today utilizes a “cored” construction technique. An outer fiberglass skin, with gelcoat, is bonded to a core material and an inner fiberglass “skin”. The core can be plywood squares, end-grain balsa, paper or plastic honeycomb or one of several different types of plastic foam. The voids between adjacent pieces of core are supposed to be filled with resin but this doesn’t always happen. This “sandwich” is stronger and stiffer than the equivalent weight of solid fiberglass.  

The boat builder now starts installing hardware, ports and hatches in that deck by cutting out the opening or drilling mounting holes through the deck and the core. Then the hardware is installed, using a marine sealant to make sure there are no leaks.  

All is well for the first several years. Then the sealant begins to break down and allows water to start soaking into that inner core. There, it can start rotting plywood or balsa or break the bonds between the fiberglass skins and the core material. Finally, when everything is saturated, water may start dripping below.

Given the high labor cost of repair, nothing lowers the resale value of a boat quicker than a spongy, wet deck. Hence, the need for strategies for sealing deck hardware.

  Foundation

The first step in sealing a deck is to deal with that core material. If your deck core is dry, you can proceed to sealing the core. If it is wood of some type and is wet, you will need to remove the wet wood and replace it before sealing the opening.  

Assuming the core is dry, remove the core material from around the hole or opening. A modified Allen key in a drill is effective in removing the core around a hole. For ports and hatch openings, a Dremel tool or router can be used. The core needs to be cut back about a half an inch around the opening for best results.  

The cavity is then filled with an epoxy/filler mixture. Filler, such as WEST High-Density filler or fumed silica is used. The fumed silica has another advantage in that it reduces the tendency of the epoxy to sag and run. Epoxy syringes are effective in filling the cavities around fastener holes.  

When the epoxy/filler hardens, the mounting hole can then be re-drilled. This time the hole passes through solid epoxy and the core is protected from water intrusion.

  Installing Hardware

The first thing to consider in installing deck hardware is the type of sealant to be used. There are at least four basic types of sealants:  

·       Silicones

·       Polyurethanes

·       Polyethers

·       Polysulfides

 

Silicones are not great adhesive sealants but must be used with certain types of plastics. Polyurethanes are terrific adhesives as well as sealants, unfortunately, once installed with polyurethane, the hardware is very difficult to remove. Polyethers are new and have very good UV resistance.

That leaves my personal choice for a deck sealant, polysulfide. Polysulfides are both good adhesives and good sealants. A good example is 3M101.

 

The most common mistake people make when installing deck hardware, besides not sealing the deck core, is to have too thin a layer of sealant. They apply sealant to the hardware, and then tighten down the hardware and squeeze out all the sealant.

 

A thin sealant layer doesn’t have the ability to move with the expansion and contraction between the deck and the hardware. In a short time, the sealant layer fails and leaks begin.

 

There are two good methods of maintaining a decent sealant thickness when installing hardware. The first uses neoprene rubber washers placed on the fasteners under the hardware. When the fastener is tightened, the hardware bottoms out on the rubber washer while allowing an adequate thickness of sealant elsewhere under the hardware.

 

The second method is a little more difficult to get right but still seals well. The edges of the fastener hole are chamfered slightly. A rubber O-ring is then placed on the fastener, sealant applied and the fastener tightened down. The O-ring does two things; it seals the fastener opening and acts as a spacer to provide the needed sealant thickness. The trick is getting the right chamfer in connection with the appropriate sized O-ring.

 

Clean Up

The time to think about clean up is before you start spreading sealant around. A little prep work will make clean up easy. First, tape around the deck where the part is to be installed with blue tape three or four inches around the item. Temporarily locate the part and trace around it. Remove the part and cut away the tape from under where the part will go.

 

Then tape around the edges of the part or fitting. Place the rubber washer or O-ring on the fastener and apply a generous layer of sealant. Use more than enough, you don’t want any voids or air bubbles in the sealant.

 

Locate the part and tighten down the fasteners. DON’T OVER TIGHTEN! The excess sealant will squeeze out from under the edges of the hardware, clean this up with tongue depressors or pop sickle sticks with the ends cut square. Once the excess sealant is cleaned up, leave it harden for a short time and then remove the tape. If you were neat and taped everything properly, your clean up is finished.

 

If you want to further tighten the fasteners, wait until the sealant has fully cured. Have someone hold the fastener on deck and only tighten the nut from below. Don’t turn the fastener from the deck side or you’ll break the sealant bond around the fastener, a sure source for leaks.

 

Conclusion

Properly done the hardware installation should be good for five years or longer. After that, start thinking about re-bedding with new sealant. If you properly sealed the core, any leaks won’t damage the deck and will show up as leaks below.

 

Protect your investment in your boat, start your deck sealing projects now. You don’t have to do them all at once, take a fitting or a port at a time. Just make sure you get them all sooner than later.

 

 

“Your deck may well be leaking below your feet.”

 

“Deck leaks can be insidious, going on for years before manifesting themselves with drips from the overhead and damp interiors.”

 

“Protect your investment in your boat, start your deck sealing projects now.”

 

  “Cored” construction as used in most boat decks.

 

The proper way of sealing cored decks.

 

Two ways of ensuring a proper sealant thickness under a piece of deck hardware.

 

The result of a serious deck leak, a dangerously corroded chainplate.

  A proper job of taping around a fitting, use plenty of blue tape!

 

 

 

 

 

 

 

 

Comments

[dave calhoun]

Great article, and I agree with everything except the use of o-rings or spacers when bedding deck hardware that will under stress, especially any sort of moment (bending) load.

O-rings or spacers create two problems:

1 - The washer/o-ring acts as a fulcrum allowing the cleat to rotate about the bolt. This causes the edge of the cleat to dig into the deck and can result in gelcoat damage or worse.

2 - A compliant layer does not allow the bolt to preload properly and the nut can work loose under repeated cycles of loading and unloading.

This also true of the common mistake of putting a thick layer of sealant and partially tightening the bolt while the sealant cures to "make a gasket".

Modern sealant does not appreciatively shrink in thin sections and as long as the sealant bead has no communication channels between the bolt hole and the outside world, the seal is complete. A thin but complete layer is all that is required. Just make sure you use an excess of sealant and see that it oozes out ALL the way around the cleat.

I'm a mechanical engineer with practical experience with this problem both professionally and messing around with boats. I learned this from experience. When I had a similar problem I discussed this with Locktite corporation, the particular product I was using at the time. They are the authority in sealants and adhesives.

Dave Calhoun
Freedomrider II
Hunter 29.5
San Francisco