Hardware that lives near the waterline and is subject to movement, like gudgeons and backstay fittings, needs to be securely fitted and mounted with sufficient bedding so that water will not penetrate the hull.
Regardless of its size, location, or the required strength, every deck fitting must be bedded for two good reasons. Decks and fittings are not perfectly smooth and flat, and will allow water to be trapped in the cracks and crevices left if they are simply bolted together. This moisture is destructive to many metal fittings and will eventually find its way inside the boat, resulting in an even more annoying and destructive leak. If the deck is built of plywood or balsa-cored fiberglass, water will penetrate the core, requiring an expensive repair in later years. Bedding compound is cheap by comparison.
Use a good-quality, marine-grade caulk from a recognized company when installing deck hardware. These products have UV and other inhibitors added that cheap, home-center products do not. Most sailors rely on 3M, Sikaflex, and LifeCaulk for these caulks. Be aware that there are three distinctly different marine caulking/bedding products on the market. Silicones have limited applications and are good where unloaded plastic parts, such as a bulkhead compass, are to be installed in the cockpit. Many silicones cure very fast and can be hard to work with. At the other end of the spectrum are the polyurethanes, which are listed as adhesive sealants. These are only for permanent bonding, such as a hull to a deck, and should not be used to bed down any piece of gear that you'll ever want to remove for replacement or repair. Lastly, the polysulfides are usually the best choice for most deck installations. Read the manufacturer's information for the item you are installing as well as the instructions on the tube of caulking—there are special considerations with some things like plastic portlights where the bedding compound may be incompatible with the fitting material. There are specialty and blended caulking compounds to suit these applications.
Bedding is a messy process when done incorrectly. The stuff can spread like wildfire, and if allowed to contact a porous fitting such as a teak handrail, caulking will be very difficult to remove. The proper way to avoid a mess is to dry fit the piece of hardware after the holes are drilled. This allows you one last chance to ensure that the holes line up, that you have all the proper backing plates, washer, nuts, and that the bolts are the proper length. When all is in good order, mask the deck around the fitting leaving just a barely-visible line of deck showing (if it is round, use lots of little overlapping pieces of tape).
Now remove the fitting, clean up your drill holes in the deck with a small countersink, and mask the edge of the fitting leaving that same hair's width of metal or plastic showing. Make sure the deck and fitting are cleaned of any drilling chips, dry, and free of any grease or oil—a swipe with an acetone rag won't hurt. Then, slather the compound on. When you put the fitting on the deck, rotate it and slide it around to ensure an even coating between them. Then insert and tighten the fasteners all but the last turn. With bolts, please note that this takes two people, so have your helper lined up first or you'll be wandering around the docks looking for help, spreading caulk along the way.
"Setting bolts takes two people, so line up your assistant beforehand or you'll find yourself wandering the docks looking for help, spreading caulk along the way."
Before the bedding that has oozed out all around the fitting has time to get stiff, assemble lots of paper towels and a waste bucket lined with a plastic bag. If you're squeamish about using your fingers to wipe away the excess caulking and shape it, wear a latex rubber glove or use a shaped tongue depressor—but nothing works as well as a bare finger and acetone will get most of it off later. Keep wiping the excess onto the paper towels until you see clean masking tape, then remove the tape (carefully and under complete control to avoid spreading caulking everywhere) and throw it in your waste can. Voilà! A perfectly clean installation. Now throw the waste bag away before it blows overboard.
Notice that you didn't turn that last bit on the fasteners. Many caulks shrink a bit as they cure. When fully mature, you will have a custom-made, form-fitted rubber gasket under your fitting. In about a week for most caulks, you can take that last turn on the fasteners and put the gasket in compression, giving you a leak-resistant joint that will last for years.
This article was originally published on SailNet in February 2001.