Boat Sealants

By Don Casey

Perfect technique can't make up for the wrong sealant. If you really want to stop that leak, start by getting the sealant right.

You've been eyeing that bow cleat all winter. You can tell there's no sealant around the base anymore, and you've noticed the dirt trails on deck and below weeping from the bolts. You're sure a leak through those bolt holes caused the annoying damp spot in the forepeak last summer. Time to stop procrastinating and re-bed that fitting. Done right, it's one of the easiest, most satisfying, and most important jobs aboard, one that will not only keep your boat dry down below, but also prevent major structural damage. Done wrong, it can destroy deck coring and cost you a great deal of money and time to fix.

Doing the job right starts with using the right sealant. Picking the wrong sealant can cause a host of problems from early failure to not being able to free a fitting if necessary. Some sealants will never bond to plastic; others deteriorate when exposed to chemicals. Choose the wrong sealant and, at best, you'll be doing this job again next year. At worst, you'll have to destroy some of your deck to free a cleat.

Unfortunately, the manufacturers don't make it easy to figure out what sealant will work best for your particular project. Well-stocked marine-supply stores have four types of sealant on their shelves — polyurethane, polysulfide, polyether, and silicone — most of which say only "marine sealant" or maybe "adhesive sealant." An additional sealant worthy of consideration will not even be on the shelf. Rather than a gooey paste applied with a nozzle, butyl tape is a sticky solid pressed into position (see sidebar).

The best-known modern marine sealant, 3M 5200, a polyurethane, has a well-deserved reputation for unsurpassed strength and tenacity that makes it the go-to sealant. But, as you'll see below, for many applications, including bedding deck hardware, another product would be a wiser choice. Formulated for cohesive failure (the sealant fails before the bond releases), 3M 5200's tensile strength of 700 psi (pounds per square inch; the force necessary to pull the bonded pieces apart) means it can take more than 5 1/2 tons of force to separate a 4-inch stanchion base from the deck. 3M 5200 is, in fact, a construction adhesive, not a sealant. A construction adhesive bonds two surfaces with a near-permanent bond; a sealant keeps water out. Strength is not the first requirement for a good sealant to bed deck hardware held in place by mechanical fasteners. Understanding what really matters will help you to pick the best alternative.

What Really Matters

A good marine sealant for bedding deck fittings must be waterproof, of course, but it must also be flexible, UV resistant, and, ideally, chemical resistant (fuel, bleach, and other solvents do find their way on deck occasionally). It should not be so strong that the deck hardware can't be removed if necessary, or so tenacious that it leaves a residue that prevents other sealants from adhering. From an aesthetic perspective, it should resist dirt and not age in an unsightly way.

The table below summarizes how the various adhesives line up against these criteria. Strength is measured in psi. Flexibility is measured by elongation, or the amount the material can be stretched, as a percentage of its original length, before it ruptures. When bonding two different materials together, and then subjecting the bond to movement, elongation allows the seal between the two materials to stretch to accommodate the differences in flexibility. A good sealant for bedding deck fittings has relatively low tensile strength and relatively high elongation.

Comparison of Retail Marine SealantsClick on table to enlarge

The high tensile strength of the medium- and high-strength polyurethanes makes them excellent adhesive sealants, but less than ideal for anything that might ever need to be dismantled. In addition, the polyurethanes will soften when exposed to fuels, solvents, and particularly acids, so they're not the best choice for use on the deck. At first glance, silicone seems a better compromise, with lower tensile strength than the medium- and high-strength polyurethanes, but similar elongation and better chemical resistance. Unfortunately, the adhesion of general-purpose silicone sealants is often fleeting; you won't have to look far in any marina to see silicone "worms" dangling from portlight frames. Worse, silicone leaves behind an all but permanent coating to which no sealant, including fresh silicone, will adhere. Contamination from some earlier silicone sealant application is often the undiagnosed cause of premature bedding failures. But silicone cannot be avoided altogether as it is one of the few sealants compatible with all plastics.

The remaining sealants all have relatively low tensile strength and high elongation, and any of them can be used for bedding deck fittings. The low-strength polyurethanes have less holding power than their more muscular siblings, but share their susceptibility to chemical attack and incompatibility with plastic (except Sikaflex 295 when used with a primer). Butyl tape is an old solution that's becoming popular once again, and it has a lot to recommend it (see sidebar). It can be used to bed all types of plastics, unlike polysulfide, which attacks plastic, and polyurethane (with the exception of Sikaflex 295), which loses its grip due to plastic outgassing.

Polysulfide resists chemical attack and is unaffected by submersion. When cured, though, polysulfide has less stretch than other marine sealants. If you tighten the fasteners enough to squeeze out all but a paper-thin gasket of sealant, the seal will rupture if opened by just a few thousandths of an inch. Good bedding technique resulting in a thick layer of sealant overcomes this problem, making polysulfide suitable for virtually all non-plastic bedding applications, including tank fills and vents, wood components that might be subjected to bleaching, and thru-hull fittings both above and below the waterline.

Polyether sealant is the new kid on the block. The only polyether sealants you'll find in marine supply stores will be West Marine's Multi-Caulk and 3M 4000 UV. The primary attraction of 3M 4000 UV is its versatility. It's the only marine sealant suitable for both above and below the waterline that brings no risk of incompatibility, whether sealing fiberglass, metal, wood, or plastic. (Note: Multi-Caulk is not safe for ABS or Lexan plastics.) 3M 4000 UV's low odor and easier cleanup make it more pleasant to use than the other curing sealants. However, with a tensile strength similar to the low-strength polyurethanes, it will deliver a stronger bond than desirable for bedding applications where future disassembly is likely.

The Bottom Line

There's much to like about butyl tape for bedding deck hardware. It takes a bit longer to apply, but it's easy, relatively mess-free, and the job is finished as soon as the nuts are tightened — no waiting for the sealant to cure. It's the best choice for framed portlights, but should be avoided where it may come into contact with chemicals. It does not cure, so butyl tape properly installed should remain watertight for decades, yet it's also the easiest to dismantle.

If you can't find butyl tape or have more faith in a curing sealant, Boatlife Life-Calk polysulfide, applied as described in the article, "Re-Bedding Deck Fittings", will be your best choice for bedding metal and wood (but not ABS or Lexan) because of its excellent chemical resistance and emphasis on sealing rather than bonding. The polyethers accommodate movement better than the polysulfides and have better UV resistance, and 3M 4000 UV is even compatible with plastic. But the stronger bond will be problematic if disassembly is required.

Sikaflex 295 UV polyurethane is another alternative to polysulfide. A combination of superior UV resistance, liberal elongation, and compatibility with plastic (in concert with a primer) makes this a versatile sealant. Its advantage over 3M 4000 UV and over all of the other polyurethane products is its lower strength, which makes future disassembly/removal easier. You can, of course, use any of the other polyurethanes, but unless your intent is to bond rather than seal, these are choices you may come to regret.

The alternative for sealing framed windows, if you skip butyl tape, is silicone sealant. Bonded windows require a structural glazing silicone such as Dow 795 (or Sikaflex 295 UV polyurethane protected with a special primer). Beyond portlights and specialized uses, you'll save yourself grief if you keep silicone sealant away from the deck and hull.

Don't just pick up any tube of marine sealant from your favorite chandlery and set to work. If you want to make sure that leak doesn't come back, take the time to select the best sealant for the job. While it may not be as much fun as playing with drills and bolts, choosing the right sealant is every bit as important as the proper technique to make that fitting watertight. 

Don Casey contributes to BoatUS Ask The Experts.

— Published: April/May 2013

An Old Sealant Makes A Comeback

Butyl tape has been used successfully for years, and has numerous advantages. With adhesive strength similar to duct tape and about a tenth of the tensile strength of even the weakest curing sealant, butyl tape doesn't harden, but remains sticky and pliant. While that makes future disassembly a snap, butyl tape is still less likely to break its seal because it stretches. If a gap does open up, the still-sticky tape self-heals.

Illustration of the underside of a deck cleat bedded with butyl tape The underside of a deck cleat bedded with butyl tape. (©2013 Mirto Art Studio)

Cutting, fitting, and molding butyl tape can be more labor intensive than squeezing on a squiggle of Life-Caulk, but tape is far less messy, eliminating the need to mask. Compatibility with plastic, combined with a gasket-like and uniform application, make butyl tape the most reliable sealant for framed portlights. Butyl tape doesn't cure, so you can take all the time you need fitting hardware. Success is less dependent on technique than with the curing sealants. It's easier to control, seems unaffected by UV exposure, is relatively low in cost, and has a shelf life measured in years rather than weeks.

All is not completely rosy, however. The malleability of butyl tape changes with temperature, making it difficult to use in cold weather and prone to compression creep in tropical heat. Permanently sticky edges capture dirt. Butyl tape is not an adhesive — it can be used only for sealing hardware bolted in place. Don't use it with screws because rotating them breaks the seal. Petroleum products dissolve butyl tape, so it is a poor choice around fuel fills or vents. Likewise, its modest adhesive strength rules it out for use below the waterline.

Even for the uses where butyl tape clearly excels, the biggest problem is picking the right butyl tape. Primarily a construction product intended for sealing siding, windows, and roof panels, it's manufactured in hundreds of varieties, comes in a coil, not a cartridge, and is unlikely to be found at a marine supply store. The usual retail source for the individual user is an RV store. Even here, expect to find more than one type. Black is a no-no because it stains, but how do you decide between the soft one and the harder one, between the thin one and the thicker one, between the gummy one and the one that feels less adhesive? If you have experience with butyl tape or you're confident that you'll know the right product when you feel it, fine. If you'd rather not blaze your own trail, there's a small supplier in Maine selling a butyl tape selected specifically for marine use that gets high praise from users (for details, see

In warm weather butyl tape is a joy to use, but if you're working in cold weather, you'll need a heat gun or a blow-dryer to warm the tape sufficiently to make it workable.

The application of butyl tape differs from the curing sealants. Here it will be better to apply the tape to the base of the hardware. Start by tearing off small pieces of butyl tape, rolling them into thin strands, and wrapping them around the bolts just below their heads to fill any gap between fastener and hardware. Insert each bolt into the hardware (but not the deck), and wrap a much thicker rolled strand of butyl tape around the bolt where it exits the hardware base. Use your fingers to compress the tape into the threads and against the underside of the base. Create a fat butyl rubber cone that will more than fill the chamfered hole in the deck.

Finally, cover the entire underside of the fitting with butyl tape. If your tape is thick, squeeze and stretch it for a thinner application. Leaving the paper separator in place can make it easier to work. Butt all joints, don't overlap them, and don't disturb the conical wraps around the bolts.

With the base covered and the separator paper removed, fit the fasteners into the holes in the deck. Because you're inserting all the fasteners at the same time, if you're using a backing plate or have an inside frame, have your helper hold it tightly against the underside of the deck as the bolts are pushed through, because small deviations from perpendicular will change the spacing of the ends of the fully extended bolts.

Fit each bolt with a washer and nut and tighten them evenly. As with curing sealants, a bead of squeeze-out should form all around the base, but you're not at risk of squeezing out all of the sealant. To the contrary, the right butyl tape will compress begrudgingly, creeping rather than squishing. That means you have to tighten the nuts several times over a span of days. There is little chance of displacing all of the rubber, but that does not mean cranking down on the fasteners. When they stop turning with wrist strength, they're tight enough.

Cut squeeze-out free with a plastic blade and peel it away. Tapping the deck with a blob of the excess will pick up stray bits. A liquid cleaner wax is safer to use for deck cleanup; mineral spirits can weaken the seal. Done properly and not exposed to petroleum, the seal butyl tape creates should be no less permanent for bedded hardware than that of curing sealants, and its superior stretch and resilience can mean a longer life.


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