Stopping The Leak

by Don Casey

Get rid of persistent leaks by removing the offending fittings and resealing them properly.

Water stains, damp cushions, or a "swamp rug" in a low corner below are signs that at least one piece of deck hardware may need to be rebedded. But which one? One surefire way to locate the source of a leak is to seal all vents, hatches, and openings (including through-hulls) and pressurize the cabin with a leaf blower or ShopVac. Soapy water sponged over suspect joints and fasteners will reveal a leak as a fountain of bubbles; this technique is similar to the way you would find a leak in a tire.

Another way to find a leak is bo build a dike around the suspect fitting with modeling clay and then fill the formed basin with water. Be patient when using this method; a slow leak can take several hours to show up below, especially if the water is being soaked up by the deck core. Check one fitting at a time. When you've located the source of the leak, don't just caulk around it--this will solve the problem only temporarily. You must remove the fitting and rebed it.

To remove a bung that hides the fastener in a wooden fitting--a handrail, for example--center-drill the bung. Then thread in a screw whose diameter is just slightly larger than that of the drill hole. When the tip of the screw encounters the head of the fastener underneath the bung, turn the screw further to jack the bung from the hole. Make sure that all varnish or sealant has been removed from around the bung; if some remains, the bung could rip when it comes out of the surrounding wood.

Another tip: Nuts are tougher than screw slots. Loosen fitting fasteners by turning the nuts underneath, not the bolts (machine screws). Have a helper hold the bolt motionless with a screwdriver that fits the bolt head. For better torque resistance, use a square-shank screwdriver and a wrench that fits the shank; this device will easily hold the torque that you apply to the nut.

Rebedding Preparation

when the fasteners have been taken out and the piece of hardware is off, remove all traces of old bedding from both the deck and the fitting. Shave off the sealant with a scraper or a chisel-point putty knife. Finish the job with a rqag and 3M Adhesive Cleaner or acetone.

Shine a flashlight on the fastener holes while you probe them with a piece of stiff wire. Any exposed core, wet or dry, must be sealed. First drill the holes 1/8 inch oversize. If the drill also brings out wet shavings, you need to dry the core. Plug the bottom of each hole and flood it with alcohol. Give the alcohol a few hours to displace the water, then open the bottoms and pass a hairdryer over the holes--not too close--to hasten evaporation. When the holes no longer smell of alcohol, the core will be as dry as you're going to get it.

Seal the bottom of the holes with duct tape, and fill them with unthickened epoxy. If the mounting hole your're working on passes through a molded headliner, you must pack something into the space between the deck and the liner to keep the alcohol and/or epoxy from running out onto the top of the liner. When the epoxy has cured, redrill the mounting holes through the center of the cured epoxy.

A thin epoxy liner around the fastener is inadequate for balsa or other soft cores. In this case run your oversized bit through only the top skin, then chuck a bent nail into the drill and pulverize as much of the core as you can reach through the hole. Push the fragments out the bottom, and dry the core if necessary. With the bottom of the hole taped, fill the widened cavity with unthickened epoxy to saturate the core around the hole. Then puncture the tape to let the epoxy drain back into the mixing container. Seal the punctures with fresh tape, thicken the epoxy to a ketchup consistency with colloidal silica or microfibers to strengthen it, and pour or inject the epoxy back into the holes until it fills the cavities. "When these compression plugs have cured, redrill the mounting holes.

Deck Camber

Because decks are curved, hardware with a flat base tends to rock, which is detrimental to sealant life. One way to add to the life expectancy of your bedding job is to countersink the holes in the deck; doing so forms a cone-shaped collar of sealant around the fastener that will compress against the shank when you tighten the nut (see "Drilling Solid Fiberglass," page 59, for the proper technique). You can double this effect by countersinking the holes in the base of the hardware, creating a sort of packing bland around the fastener. Make sure, however, that the hole has sufficient depth to allow you to countersink it without also enlarging its diameter.

While there isn't much you can do about metal fittings, investing five minutes in hollowing the bases of wooden fittings could save you hours of future rebedding. The best way to get a uniform hollow is to pass the wooden fitting diagonally across a table saw with the blade set just above the table. First test the wooden fence and blade settings on a scrap of wood that has the same width as the deck fitting. About 1/8 inch on either side of the fitting base should be left untouched by the saw blade (Fig. 2).

The Right Sealant

Polysulfide is the most versatile marine sealant and adheres well to metal, fiberglass, and evern (with the help of a special primer) to the oily surface of teak. Polysulfide’s only limitation is that it attacks plastic, so you cant bed ABS hardware or plastic portlights with it.

Although silicone is also touted as a good adhesive, it's not perfect. However, its and unbeatable gasket material when it's put under compression. Silicone lasts indefinitely, is impervious to almost everything, has excellent insulating properties, and is friendly to most plastics, including acrylic and polycarbonate.

Polyurethane, which is the third member of the marine sealant trio, is beyond reproach as an adhesive. In fact, it's so tenacious that you should consider anything bedded with polyurethane as bening permanently attached. Because almost everything bolted to a sailboat deck, other than the hull, will need to come off eventually, I wouldn't use polyurethane to bed deck hardware.

BoatLife's LifeSeal is a combination of silicone and polyurethane; it has all of the desirable characteristics of silicone but with better adhesion. It is a superior sealant for plastic portlights and is also excellent for bedding metal hardware. It's not as good as polysulfide bor bedding wood (See "Sealant Choices" below).

Sealant Choices

Type Use
Polysulfide The best choice for most deck fittings except plastic. Adheres well to primed oily woods.
Polyurethane A permanent adhesive. Not for fittings that may need to be removed or for plastic fittings.
Silicone A weak adhesive, but forms chemical-resistant gasket. Safe for plastic fittings.
Silicone/Polyurethane Same characteristics as silicone, but with better adhesion. Excellent for portlights.

Using Sealants

To avoid a mess always mask the area first. Tape the mounting location and go an inch or so beyond it; use 3m's Long Mask (blue )tape. With a razor blade, trace around the fitting and remove the outlined tape. Also mask the sides of the fitting.

Apply an even coat of sealant onto both surfaces to make sure there won't be any gaps in the bond. Put the fitting in place but give each bolt a narrow ring of sealant just below its head before inserting it. Wipe off all sealant pushed through the deck by the bolt; if the seal on deck fails, you want the water to drip into the cabin, not back up into the core.

Have your helper keep the bolt heads from turning; snug up the nuts until the sealant squeezes out around the fitting, then stop! The biggest mistake you can make is to overtighten the fitting; doing so squeezes out all the sealant and leaves the joint with no elasticity. Let the sealant cure for 24 hours (1 hour is sufficient for silicone), then with your helper keeping the bolts from turning, tighten the nuts some more. This will put the joint under compression, so even if the sealant's adhesion fails, its gasket will continue to provide an effective seal.

Finally, trim off excess sealant by running a razor blade around the fitting, and peel up the masking tape. A thin line of sealant beneath the fitting should be the only evidence of your work--except that now everything below will be dry.

Don Casey, the author of This Old Boat, is a frequent SAIL contributor.

[Posted with permission from SAIL.]