Stop That Leak!

Modern chemistry has presented us with new choices of sealants for everything, including the kitchen sink.


The trick is to choose the right one. Some sealants get hard enough to sand or drill, and others stay supple. Some will stick to anything; others pull away from glass and certain plastics. Some will writhe and stretch as your boat “works.” Others crack. And some, designed for household use, won’t hold up in the harsh heat, cold, wind, and ultraviolet your boat is subject to.

There are four major sealant types, all with special uses (five, if you count polyurethane, which is really an adhesive rather than a sealant). Read the labels to be sure, but a good marine sealant should perform as follows:

Sticks to almost everything including glass, electrical insulation, and most metals. Ideal insulator and waterproof for wiring including trailer wiring, windshields and ports, and emergency gaskets in applications where temperatures don’t exceed 400 degrees F. Don’t use with polypropylene, under water, or in areas where you want to sand and paint. Not as good as polysulfide in areas that take a lot of twisting, compression, contraction, and expansion.

Use above and below the waterline. Can take up to 25 percent stretching, twisting, expansion and will bond difficult surfaces including oily woods, aluminum, glass. (Read directions for possible surface preparation steps.) Can be sanded and painted. Available in liquid form to ease filling of hairline cracks.

Drawback: takes up to 10 days to cure in a humid climate; longer in dry climate. If you’re not counting on it as a waterproofer, you can launch the boat right after caulking because water speeds curing time. Other caulks cure faster

Cure fast, stick well, can be used on polypropylene where polysulfide cannot. Not sandable, but can be painted. Easy to apply, and cures to supple rubber.

Ideal for bedding on wood, fiberglass, metals. Skins over quickly and can be painted after half an hour; full cure in one to two days. Water soluble which means easy, water clean-up but also that it’s not suitable for underwater use.

Hints on application:

  • When trying to form an even, good-looking bead with polysulfide, coat your fingers with liquid detergent and you can mold the sealant, after it skins over, without sticking.
  • When using a caulking gun, push, don’t pull.
  • Polysulfides cure faster when wet. To speed up curing, adjust hose nozzle to fine spray and keep sealant damp. For smaller areas, use a spray bottle.
  • Sometimes a primer is needed before applying sealant to some surfaces. Read labels carefully so you can buy needed primers and solvents before leaving the store.
  • It’sbest to avoid caulkingincold weather. Roth the caulk and the boat should be in moderate temperatures for the best result.
  • Use underwater caulk on through-hulls to insulate, not just against leakage but against galvanic corrosion.
  • Marine polysulfide sealants help keep engine mounting bolts from corroding or vibrating loose, and ease future removal. Put some on the threads before turning the nuts down.
  • If you carry an extra tube of marine silicone sealant aboard, you can jury rig any size gasket.
  • When making a hatch gasket from silicone sealant, placeabeadonbothsurfaces, thencoverwithwaxed paper and close hatch. Paper keeps gaskets separate as they cure.
  • Coat back sides of light fittings to seal wire ends (but not light bulb socket) to prevent corrosion.
  • For proper adhesion, seams in teak decking should be at least l/&I-inch wide, l/4-inch deep.
  • -By Janet and Gordon Greene

    Stop That Leak!

Practical Sailor has been independently testing and reporting on marine products for serious sailors for more than 45 years. Supported entirely by subscribers, Practical Sailor accepts no advertising or any form of compensation from manufacturers whose products we test. Testing is carried out by a team of experts from a wide range of fields including marine electronics, marine safety, marine surveying, sailboat rigging, sailmaking, engineering, ocean sailing, sailboat racing, and sailboat construction and design. This diversity of expertise allows us to carry out in-depth, objective evaluation of virtually every product available to serious sailors. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser with more than three decades of experience as a marine writer, photographer, boat captain, and product tester. Before taking on the editor’s position at Practical Sailor, Darrell was the editor of Offshore magazine, a boating-lifestyle magazine serving the New England area. Darrell has won multiple awards from Boating Writer’s International, including the Monk Farnham award for editorial excellence. He holds a U.S. Coast Guard 100-ton Master license and has worked as a harbor pilot and skippered a variety of commercial charter boats.


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