PracticalBoater
Skills | Techniques & Best Practices

 

Boat Sealants

By Don Casey
Published: April/May 2013

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

Photo of various brands of sealant

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.

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