The terms marine caulk and marine sealant are often used interchangeably in boating circles. The majority of marine-specific sealants are one of three material types: Polysulfide; Polyurethane; and Silicone. A fourth kind of sealants are hybrid silicone and polyurethane compounds that provide benefits of both materials. These materials are designed for different uses, and using the wrong one can cause poor results. By considering the task and the materials you need to bond, and each sealants properties, it is easy to choose the right sealant.
Caulks can include polysulfides, polyurethanes, silicones, hybrids, and butyl tape. Traditional wooden boat builders use other products referred to as caulking but are generally not liquids dispensed from a cartridge or tube. Examples of classic wooden boat caulk items include Oakum, Caulking Wick, Caulking Cotton, Caulking Irons, Bedding Compounds, Seam Compounds, and Seam Cement. These are used for above-waterline and below-waterline planking applications.
Marine sealant creates an airtight, watertight seal between multiple surfaces. Specially formulated sealants isolate surfaces and protect against vibration, noise, or electrolysis. In fact, these flexible sealants can withstand some degree of the bonded surfaces movement, and will not lose their adhesive properties.
Other sealants function as an adhesive, with additional support from mechanical fasteners. Sealants often have impressive bonding strength, sometimes approaching permanent seal status. Depending on the properties of the sealant, it can cure in hours, or take a week or longer. Some sealants cure the fastest in moist environments (e.g. underwater).
This is the most important question: what kinds of materials will you bond or seal together? If there are two or more dissimilar surface materials, ensure that the sealant is compatible with each one.
If you think the bond will be temporary, use a compound that can be de-bonded if necessary. If you are rebedding deck fittings, for example, choose a compound that you can remove fairly easily when you must repeat the process.
On the other hand, if you want the bond to last for years, 3M 5200 is the gold standard. Still, it is possible to de-bond surfaces treated with this legendary sealant. Anti-Bond 2015, supplemented by some strategic bond scoring, should do the trick.
Sealants that contain water-absorbing components will likely fail if they are underwater for a considerable length of time. Choose polyurethane adhesive sealants or elastomeric sealants, both of which are designed for above-waterline or below-waterline use.
Marine sealants take different amounts of time to cure. In fact, the most effective products require the longest curing times. For example, 3M 5200 requires 5-7 days of curing time. Life-Calk, often used to bond deck seams, takes 10-20 days to cure completely.
Polysulfide caulks are multipurpose marine sealants with many applications above and below the waterline. These flexible adhesive compounds are ideal for teak decks, bedding deck and hull hardware, sealing thru-hull fittings, and underwater seams. They bond to fiberglass, wood, metal, glass, and themselves.
Polysulfides (for example, Boatlife Life Caulk) exhibit excellent resistance to teak oils, gasoline, and diesel fuel. Note that some can take a week to fully cure. Upon full cure, they form a firm, flexible rubber seal with excellent waterproofing and adhesion qualities. These sealants have good-to-excellent resistance to UV light and petroleum products, and may be painted over.
Polysulfides contain solvents that can damage ABS, PVC, acrylic and polycarbonate plastic surfaces. Do not use for bedding plastic windshields, plastic portlights (acrylic aka Plexiglas; or polycarbonate aka Lexan), or bedding plastic marine fittings made from ABS or PVC.
Polyurethane adhesives form an extremely strong bond while retaining flexibility and strength above or below the waterline. Polyurethanes are ideal for thru-hull fastening, and the best choice for hull-to-deck joints, deck fittings, railings and transducers. They bond tenaciously to fiberglass, wood, metal, and glass. Typical cure times are a week or less.
Important notes: Do not use when bonding parts that may require disassembly in the future, as polyurethanes are generally permanent. Do not use to bed plastic windshields, plastic portlights (acrylic aka Plexiglas; or polycarbonate aka Lexan). Alcohol should not be used when preparing surfaces for bonding as it will stop the curing process and cause the adhesive to fail. Polyurethane is not recommended for use as a teak deck seam sealer. Extended exposure to chemicals (teak cleaners, oxalic acid, gasoline, strong solvents and other harsh chemicals) may cause permanent softening.
Examples of polyurethanes include: 3M 5200, 3M 5200 Fast Cure, 3M 4200 Fast Cure; Sikaflex 201US, 252, Sika-292, Sika-296, Sika-295UV, Sika-521UV, Sika-291, Sika-221.
Elastomeric sealants create a waterproof seal that make them a good option for stopping leaks, sealing portholes, and bedding hardware and windshields. They are generally compatible with fiberglass, wood, glass, and metal surfaces, but should not be used on deck seams.
Unlike other sealants, elastomerics can be used on plastics and Plexiglass. They cure fast, slightly more than one day, have high UV stability, and are paintable and stain resistant. And, because it is not a permanent adhesive, you can disassemble components if necessary. Because they are intended for bedding and sealing, they lack the bond strength of polyurethanes.
Examples of elastomeric marine sealants include: TotalBoat Seal, Sikaflex 1A, 3M 3000 UV.
Marine Silicone sealants are mildew resistant, non-sagging, moisture curing products, suitable for above the waterline applications only. Silicones remain flexible, with excellent resistance to the corrosive effects associated with most chemicals, salt, dirty, spray, and moisture.
Use on bare and painted metal, glass, fiberglass, non-oily woods, many plastics and abraded rubber. Silicone sealants typically set in a few minutes while reaching full cure in a day.
Examples of suitable applications include head plumbing, interior & exterior wood, trim sealing and as a gasket adhesive. Do not use silicone sealants to bed or install glass, polycarbonate (Lexan) or acrylic (Plexiglas) windows that are not also mechanically fastened. Do not use silicone on any surface to be painted.
Examples of marine silicones include: 3M Marine Grade Silicone Sealant; Boatlife Marine Silicone Rubber
Teak deck caulks are typically one-part, neutral curing, adhesive sealants specially formulated for caulking teak decks with outstanding adhesion to teak and other naturally oily woods. When used as directed, no primer is necessary. The caulk cures when exposed to moisture in the air to form a tough, flexible, solid rubber seal with excellent temperature stability and resistance to chemicals, sunlight, UV and water. Upon full cure, it can even be sanded flush.
Note: Cure can take up to two weeks. Do not use for applications outside of caulking teak deck seams.
Examples of teak deck caulking include: TDS Teak Deck Caulking; Maritime Teak Deck Caulk
Traditional wooden boats are constructed and maintained with these 'old time' materials. The caulking on wooden vessels uses fibers of cotton and oakum. Fibers are driven into the wedge-shaped seam between the planks with a caulking mallet and a broad chisel-like tool called a caulking iron. The caulking is then covered with a seam compound in the case of hull seams, or else in deck seams with melted pine pitch, in a process referred to as paying. Modern marine sealants are frequently used now in place of the pitch, or even to supplant the oakum and cotton.
Examples include: Oakum, Caulking Wick, Caulking Cotton, Caulking Iron, Dolfinite Bedding Compound, Pettit Seam Compound, Interlux Boatyard Bedding Compound, Davis Slick Seam.