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
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.
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:
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
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