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A leaking deck fitting can cause damage to deck cores and boat interiors. But rebedding may be all that's needed to keep the water out.
Cleats, deck fills, lights, handrails, and winches are just a few of the many deck fittings found on the average powerboat or sailboat. All usually require drilling holes to attach them, and all are usually bedded into some sort of compound when a boat is built. Over time, age, sunlight, water, and general wear and tear take their toll, and when the bedding starts to break down, water is able to find its way below or, worse, into deck-coring materials. (See the section “Dealing With Cored Decks” below.)
If you suspect that a fitting is leaking, smearing a bit of goo around the outside and hoping for the best won't cut it. You've got to bite the bullet, remove the fitting, and start fresh. Removing old bolts and screws that may have corroded is often the worst part of the job, but has to be done. Having a helper hold the bolt while you remove the nut from below will make the job go quicker, and having a little moral support won't hurt, either. First, let's look at how to rebed a fitting that may be leaking or loose.
Rebed An Existing Fitting
1. Smaller fittings may be held in place with screws, in which case you shouldn't need to get to the underside of the deck. (Consider, however, whether the screws are appropriate for the fitting and decking and whether replacing screws with thru-bolts and backing plates would be better.)
2. When the bolts or screws are removed, the fitting should come away from the deck. Pry it up with a putty knife or other thin blade if needed, careful not to damage the fiberglass. At this point, you'll be thankful if the original installer used the correct bedding compound. (See "Bedding Compounds" below) If an adhesive sealant such as 5200 was used, getting the fitting up can take longer, although there are products, such as DeBond, specifically intended to help with this issue. Don't rush if the fitting doesn't pop off right away; carefully work a blade underneath to separate it from the deck.
3. Remove the old fitting.
4. Clean all traces of the old compound from the mating surfaces on the underside of the fitting and deck, then wipe with a solvent such as acetone to remove grease and other contaminants.
5. Tape off the work area to protect surrounding surfaces, and apply a generous but not excessive amount of the chosen bedding compound to the area under the fitting. Don't just squirt a bead around the perimeter; cover the whole base.
6. Place the fitting in position, insert the bolts or screws, and tighten evenly. Use new fasteners if the old ones are corroded, the heads are chewed up, or they're suspect in any way.
7. If you applied the correct amount of compound, some should squeeze out all around the base, a good sign. Scrape up as much excess as possible.
8. Finally, wipe with the solvent recommended by the manufacturer.
Bedding Compounds
Marine stores present you with a bewildering array of mastic and sealants. Knowing which one to choose is key. The most common products available are either polyurethane or a polysulfide, both of which are excellent — but only if used for their intended purposes.
Polyurethane is an adhesive compound. 3M 5200 is perhaps the best-known product in this category. It shouldn't be used when there's a chance that you'll need to dismantle the joint later. Many manufacturers use 5200 for hull-to-deck joints, and it's perfect for that kind of application. Polyurethanes hold up well underwater and bond to most materials, but they can attack some plastics. They don't harm Marelon, so it's OK to use for bedding underwater fittings made of this material.
Polysulfides, such as BoatLIFE products, remain permanently flexible. These are my preferred choice for most jobs. They can be used above and below the waterline, and the joint can be separated later if necessary. Polysulfides are available in a variety of colors. I've found that the brown is a good match for teak and mahogany, but use the recommended primer first for the best seal with oily woods such as teak. A polysulfide product shouldn't be used for bonding polycarbonates such as Lexan or PVC because it will attack them. Most window frames and other components contain plastic so are best bonded using a silicone product.
Another product I favor is butyl tape. Popular many years ago, its use waned when polyurethanes arrived on the scene. Butyl tape never sets and remains flexible forever, cleanup is easy, and because it comes on a roll, you use only as much as you need. In many cases, it's more economical than the stuff in a tube.
But butyl isn't the right choice for every application. While it works well under cleats and stanchions held in place with bolts, it's less suitable under screws. Tightening the screw tends to drag at the butyl and makes it pill up, pulling it away from the fitting. Butyl works well with bolts in situations in which they can be prevented from turning and are secured from beneath with a nut. The one caveat is that not all butyl tape is created equal. Use tape that's recommended for marine use.
Even more old-school is Dolfinite bedding compound, which can be somewhat hard to find. I like it for applications in which I'm bedding wood to wood. It can also be used for hardware-bedding applications. Dolfinite remains flexible for years, won't dry out, and won't destroy the parts should you ever need to break the bond. A can of Dolfinite (available in white, gray, brown) lasts for years.
Dealing With Cored Decks
When boatbuilders first began using fiberglass, almost all boats were built from solid laminates. As the years passed and builders began to better understand the material, they incorporated coring into decks. Cored decks — now the norm rather than the exception — are lighter, stiffer, and have significantly better thermal and acoustic insulation. However, water that finds its way into the core can cause myriad problems, so it's critical that hardware is installed properly.
Cored decks are a sandwich of layers: the outer layer of fiberglass, which may have a nonskid pattern or other features and is the part everyone sees; a core, which may be one of several materials but often is end-grain balsa; and a thinner interior layer of fiberglass, which often forms the cabin interior overhead.
Balsa is an ideal core material in many ways: It's comparatively cheap, very lightweight, stiff, and offers some measure of thermal and acoustic insulation. Its one major downfall is that it will suck up water at an alarming rate. Ensuring that deck fittings are correctly installed and bedded will do much to prevent water from getting into the core material.
Visit BoatUS.com/Stubborn-Fittings for advice on dealing with stubborn fittings.
Fittings simply bedded into some form of mastic compound that is expected to keep the water out is insufficient. While it might work for a time, eventually all bedding compounds break down to some degree. As soon as the smallest cracks develop in the bedding, water will find its way into the core. Water that comes into contact with a balsa core may not initially have much effect because the water is absorbed into the wood. As more and more water finds its way into the core over time, it combines with the air in the cells of the balsa. Mold starts to grow, and the core eventually breaks down.
Problems are further exacerbated when moisture in the core freezes and expands the deck, forcing the core and laminates apart and allowing more space for water to enter as the temperature rises. As the temperature drops, everything freezes again and so on, each time doing a little more damage. Even though core material is very stiff for its weight, bolting down a winch or cleat willy-nilly crushes the core and deforms the deck. Fitting anything to the deck needs to be done properly and takes time. This initially might seem overkill, but you can rest easy knowing you've done everything possible to avoid problems and preserve the structural integrity of the deck.
1. Use a bent nail to ream out the core. 2. Drill the epoxy-filled hole for the bolt. 3. Tighten bolts — snug but not overly tight.(©2021 Mirto Art Studio (3)
There are other ways to install deck hardware, but the methods mentioned here have served me well. Here's how:
A. First, decide where to put the fitting. If it's heavy, it probably will stay in place by weight alone. Small deck cleats and those on vertical surfaces might require a piece of double-sided tape if you need to stand back to check placement. When you're happy with placement, mark the position of the bolt holes with a marker or pencil so you can easily see where to drill the mounting holes.
B. Using the marks as a guide, drill 1/16-inch pilot holes through the deck. This serves two purposes: You can go inside the boat or look under the deck to be sure holes and bolts won't interfere with anything. Plus, drilling the larger holes will be easier and more accurate because the drill has something to follow. If you have to make adjustments after drilling the small holes, filling them with epoxy is no big deal and will make them almost unnoticeable.
C. If everything checks out, carefully drill a hole twice the diameter of the bolt hole through the top layer of fiberglass and core only. (You'll feel it when you're through core and reach the inner fiberglass skin.) This hole will be filled with epoxy and will seal the core and offer solid posts that won't compress as the fastener holding the fitting is tightened. For the neatest appearance, the epoxy should not be visible around the edge of the fitting after it is installed.
If it looks as if this will happen — a few measurements can confirm it — an alternative method is to drill the correct size clearance hole for the bolt, though still going only through the outer laminate and core. Then cut the head off a nail, bend the last inch 90 degrees, and tighten it in the chuck of the drill (see Figure 1). Feed this into the hole and grind away, removing the core as an undercut.
D. With the holes drilled, put tape on the underside to prevent epoxy from running through the small pilot hole. Mix some unthickened epoxy and brush a little into the hole. You're not filling the hole at this stage, just wetting the surface as a sort of primer. With this done, carefully mix colloidal silica into the remaining epoxy until it is the consistency of thick mayonnaise. (Be sure to wear a respirator for this!) Force the mixture into the hole, using a small scraper knife or other suitable tool. Make sure there is no trapped air and that the epoxy is level with the surrounding deck surface. Allow it to cure for 24 hours.
E. Remove the tape from the underside of the deck. Using the pilot-hole mark, drill a small hole back through the cured epoxy. Then, from the top, drill down using the correct size bit for the bolt hole (see Figure 2).
F. When all holes are drilled, insert the bolts and dry fit (see Figure 3).
G. If everything lines up and looks good, add suitable caulking to the underside of the fitting, reinstall it and the bolts, and tighten the nuts so they're snug, not overtight.
Easy Masking Tip
Here's a way to save time and make cleanup a snap.
1. (Not shown) With the fitting sitting over the fixing holes, draw a pencil line around its base, then remove the fitting and put to one side. Stick down masking tape so it covers the pencil marks.
2. Replace the fitting in the correct location, then cut around the base with a sharp knife or razor blade.
3. Remove the fitting, and peel up the tape on the inside of the cut line.
4. When you bed the fitting down, any excess bedding compound will squeeze out onto the tape and will be removed when you peel up the tape. This method is faster in the long run and produces very little mess.