Skills | Techniques & Best Practices


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.

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
©2013 Mirto Art Studio, The underside of a deck cleat bedded with butyl tape.

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.End of story marker

Published: April/May 2013

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