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 poly-sulfide 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 the trademark for the polymer that is the main ingredient of all polysulfide sealants regardless of manufacturer.
If you want the two components to be joined together permanently, use polyurethane.
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.