Thread: Choosing and Using Sealants
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Old 01-22-2003
Don Casey Don Casey is offline
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Choosing and Using Sealants


You wouldn't want your bunk located under this leaking portlight, which is classic evidence of a poor caulking job. 
When we assemble—or reassemble—the various parts of a sailboat, we generally want the junctions to be watertight. For centuries sailors relied on pine tar for waterproofing. While undeniably effective, tar is also exceedingly messy. Sometime last century tar was replaced for "yacht" use by more civilized petroleum-based compounds. Dolfinite is perhaps the best known of these. Looking and handling exactly like peanut butter, Dolfinite is still available and still useful where adhesion is not desirable.

The problem with petroleum-based compounds, however, is that they dry out, inevitably losing their effectiveness over time. They have been superseded by polymers—chemical compounds that never dry out. Modern sealants, properly applied and left undisturbed, should maintain their watertight integrity for the life of the boat.

So why do leaks occur? The usual cause is mechanical failure, meaning that the bedded components pull apart under strain because of inadequate stiffness or poor attachment. Rebedding provides only a temporary respite; the permanent solution is

oversize backing plates and stronger fastenings. The number two cause of bedding failures is insufficient sealant. This we will get to, but first let’s resolve the issue of what sealant you should be using.

The confusing array of sealants and caulks available on today’s market can make one nostalgic for the days when the only choice was pine tar. Relax. Choosing the right sealant is a lot easier than you might think. Marine sealants fall into one of just three categories. Understand what makes each of these the best choice for some jobs and unsuitable for others and you have the selection process bagged.


Let the gooping begin. Modern day marine bedding compounds can work miracles against the many stresses of the marine environment, provided a few simple selection criteria are followed.
Polysulfides    You can—and should—use polysulfide to bed almost everything. A synthetic rubber with excellent adhesive characteristics, polysulfide is the most versatile of marine sealants. As a bedding compound it allows for the movements associated with stress and temperature change, yet maintains the integrity of the seal by gripping tenaciously to both surfaces. It even adheres to oily teak and is unaffected by harsh teak cleaners, making it the choice for bedding teak rails and trim. It is also an excellent caulking compound since it can be sanded after it cures and it takes paint well. (The black caulking between the planks of a teak deck is invariably polysulfide.)

One caution: do not use polysulfide to bed plastic—as in deadlights, portlight frames, or deck fittings. The solvents in polysulfide will cause acrylic, polycarbonate, ABS, and PVC to harden and split. Only when you know for certain that a plastic fitting is made of epoxy, nylon, or Delrin can you safely bed it with polysulfide. Below-the-waterline through-hull fittings fall into this group, but if you have any doubt, use another sealant.

Polyurethane     Consider polyurethane an adhesive rather than a sealant. Fittings bedded with polyurethane typically cannot be separated without damage, so do not use it on anything you might need to dismantle in the future. Polyurethane is an excellent sealant for hull-to-deck and hull-to-keel joints and a good choice for through-hull fittings, rubrails, and toerails. Do not, however, bed teak rails with polyurethane because teak cleaners damage it. Like polysulfide, polyurethane should not be used on acrylic, polycarbonate, PVC, or ABS-based fittings.

Silicone     If you think of silicone as a gasket material instead of a sealant you can proably intuit its appropriate uses. It is the best choice for bedding components that must be periodically dismantled. Its excellent insulating properties make it ideal for bedding dissimilar metals—stainless hardware on an aluminum spar, for example. And it is—by default—the only one of the marine sealant trio than can be safely used to bed plastic. However, silicone should not be used below the waterline. And because it depends upon mechanical compression to maintain its seal, silicone is also a poor choice for sealing hardware on a cored deck.

Keep this quick review in mind when you’re considering a bedding compound:


The type of surface, and whether the caulk is functioning as an adhesive, as in the case above, or a sealant all play a role in choosing the right material.
Hybrids    Normal stresses tend to pull or pry bedded components apart. If the bedding compound is both adhesive and elastic, it will stretch like the bellows joining the two sides of an accordion. Cured silicone sealant exhibits amazing elasticity, but its adhesion is temporary at best. Movement between bedded components tends to break the seal. This is precisely why normal expansion and contraction soon enough lead to leaking deadlights.

Either polysulfide or polyurethane would provide a more dependable seal for deadlights, but polysulfide is certain to attack the plastic, and polyurethane prohibits any future disassembly. One alternative is a hybrid sealant—part silicone and part polyurethane. Marketed by BoatLife as Life Seal, this mixture promises a longer-lasting seal for deadlights and other plastic fittings where compression of the sealant cannot be assured.

It used to be that rebedding was an annual chore, but today if you select the right sealant and apply it correctly, you can do the job once and forget it. Now if the test-tube wizards could just do the same for varnish.


Working the Goo

None of the sealants mentioned above will work well unless you give them a chance. If you crank down on the mounting bolts until you squeeze all the sealant out, you might as well put the parts together without sealant. The correct mounting technique is the same regardless of the sealant you use; follow these tips:

Both surfaces must be clean and dry, which means peeling or scraping away every bit of old caulking and wiping the surfaces with acetone or MEK. If you try to caulk right over the old caulking, it will leak—guaranteed.

Countersink fastener holes. Countersinking the deck allows for a wider bead of sealant around the fastener. In addition, the wedge shape of the countersink makes this "ring" of sealant function much like an O-ring when compressed. This effect is doubled by also countersinking the underside of the bedded component when thickness allows.

"There is no reason to be miserly; the sealant you save is almost certain to harden in the tube or cartridge before its next usage anyway."
Apply the sealant liberally. If it does not squeeze out all the way around the joint, take it apart and do it again. There is no reason to be miserly; the sealant you save is almost certain to harden in the tube or cartridge before you use it anyway.

Mask both components before bedding. If you are using silicone, allow the squeeze-out to fully cure, then slice it free with a razor blade. For the more tenacious poly sisters, smooth the ooze with your finger, then peel the tape promptly to leave a neat edge. Remember to seal around the fasteners. Snug but do not tighten the mounting screws. Compressing all the sealant out of the joint is the most common bedding error and it dooms the effort.

It is essential to leave the components separated by a gasket of sealant. Put the seal under compression. Let the sealant fully cure—this takes a week or longer for most polysulfides and polyurethanes—then go back and evenly tighten all the fasteners. This ensures a watertight seal even if the sealant loses its grip. Turn only the nuts on through-bolts to avoid breaking the seal around the fastener.



Suggested Reading:

Zen and the Art of Sailboat Maintenence by Don Casey

Drilling and Filling Holes in Your Boat by Sue and Larry

Mounting Deck Hardware by Tom Wood

Last edited by administrator; 08-01-2006 at 04:40 PM.