In every Marine Center you will find an array of different sealants and caulks sufficient to make your head spin. With so many choices, how do you know which one you need?
It is not as difficult as you might think. Virtually all modern marine sealants fall into one of just three types, each with specific characteristics that make it the best choice for some jobs and unsuitable for others. Selecting the right sealant is essentially a matter of identifying the materials you are wanting to seal--specifically if any component is plastic--and of determining the likelihood of ever needing to separate these components.
If neither component is plastic and if you want to preserve your ability to disassemble the joint, use polysulfide.
Polysulfide is the most versatile of marine sealants. It is a synthetic rubber with excellent adhesive characteristics, and you can use it for almost everything. As a bedding compound it allows for movements associated with stress and temperature change, yet maintains the integrity of the seal by gripping tenaciously to both surfaces. It is also an excellent caulking compound since it can be sanded after it cures and it takes paint well.
However, the solvents in polysulfide sealant attack some plastics, causing them to harden and split. Specifically, you must not use polysulfide to bed plastic windshields or plastic portlights--either acrylic (Plexiglas) or polycarbonate (Lexan). Don't use it to bed plastic deck fittings either, including plastic portlight frames. Plastic marine fittings are typically ABS or PVC, and polysulfide will attack both. If you know that the plastic fitting is made of epoxy, nylon, or Delrin, you can safely bed it with polysulfide. Below-the waterline through-hull fittings are in this group, but when there is any doubt, select an alternative sealant.
Polysulfide adheres well to teak (a special primer improves adhesion), and is unaffected by harsh teak cleaners, making it the best choice for bedding teak rails and trim. The black caulking between the planks of a teak deck is invariably polysulfide. For this application, a two-part polysulfide gives the best results. Polysulfide is the slowest curing of the three sealant types, often taking a week or more to reach full cure. Because it will adhere to almost anything, polysulfide has a maddening propensity to get on everything, so neatness is called for in using this sealant. Polysulfide sealants will have polysulfide printed on the package, or sometimes Thiokol--the trademark for the polymer that is the main ingredient of all polysulfide sealants regardless of manufacturer.
Think of polyurethane as an adhesive rather than a sealant. Its grip is so tenacious that its bond should be thought of as permanent. If there seems to be any likelihood that you will need to separate the two parts later, do not use polyurethane to seal them.
Polyurethane is the best sealant for the hull-to-deck joint. It is also a good choice for through-hull fittings and for rubrails and toerails, but not if rails are raw teak because some teak cleaners soften it. Like polysulfide, polyurethane should not be used on most plastics--acrylic, polycarbonate, PVC, or ABS. The cure time for polyurethane is generally shorter than polysulfide, but still may be up to a week.
For bedding plastic components or where insulation is desirable, silicone is the default choice.
Calling silicone a sealant is something of a misrepresentation. It is more accurate to characterize it as a gasket material. If you accept silicone's adhesive abilities as temporary, you will find it is the best product for a number of sealing requirements. It is the only one of the marine sealant trio than can be safely used to bed plastic. It is an excellent insulator between dissimilar metals--use it when mounting stainless hardware to an aluminum spar. It is the perfect gasket material between components that must be periodically dismantled--beneath hatch slides, for example.
Silicone retains its resilience for decades and is unaffected by most chemicals, but it should not be used below the waterline. Because it depends upon mechanical compression to maintain its seal, silicone is not a good choice for sealing hardware on a cored deck. Exposed silicone is a magnet for dirt and repels paint, so never fillet with silicone, and don't use it on any surface you plan to paint. Silicone sealants typically set in a few minutes and reach full cure in less than a day.
For an adhesive seal of plastic components, select a silicone/polyurethane hybrid.
An adhesive sealant maintains its seal even when stresses pull or pry the bedded components apart. The sealant stretches like the bellows joining the two sides of an accordion. This accordion effect can be especially useful for plastic portlight installations where the portlights are captured between an inner and outer frame. Although silicone has amazing elasticity, its lack of adhesion means any expansion of the space between the frames is likely to cause the seal to fail.
Either polysulfide or polyurethane would provide a more dependable seal, but polysulfide is certain to attack the plastic, and polyurethane prohibits any future disassembly. The answer to this dilemma is a hybrid sealant--part silicone and part polyurethane. Marketed by BoatLife as Life Seal, this mixture promises a longer-lasting seal for portlights and other plastic fittings where compression of the sealant cannot be assured. Hybrid Silicone/polyurethane would be best for bedding portlights that may not be under very good compression.
Butyl rubber good for compression fitting. More resilient to chemicals. Will not harden. Can be removed. Is more elastic. (Welcome To MarineHowTo.com Photo Gallery by Compass Marine How To at pbase.com