TECHNIQUES & BEST PRACTICES BY DON CASEY
HOW TO PICK THE RIGHT SEALANT
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 rails 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, page 84).
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 on page 83 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