One thing you have to do on boats is bed hardware. This has to be done on a regular basis, especially with hardware that is under heavy cycling loads, like cleats. However, using the right sealant can make this task much simpler.
Most people are familiar with silicone caulk, since it is the most commonly used type of caulk in normal household repairs. However, while certain Silicone-based bedding compounds are excellent for bedding ports, most of the time, silicone caulks really serves no useful purpose on a boat beyond bedding ports and covering the ends of cotter pins.
In most cases, on a boat, you want a sealant that will adhere to the two surfaces being joined—whether it is a through-hull and the hull or a cleat and the foredeck. The sealant should have a fair bit of elasticity so that it can stretch when the hardware moves or shifts under load without detaching from either side and breaking the seal.
For deck hardware and through-hulls, countersinking the fastener holes is probably one of the best things you can do, since it gives the sealant a natural place to form an o-ring like seal.
There are four major classes of bedding compounds/sealants used on a boat.
- Polyurethane-based sealants like 3M 4200 and 5200, SikaFlex 291, 292, 295, 296
- Polysulfide-based sealants like LifeCaulk and 3M 101
- Silicone-based sealants, like Dow 295
- Butyl Rubber Glazing tape—this is not butyl rubber caulk
Each of these sealants has specific pros and cons.
Polyurethane-based sealants are basically adhesives with sealant properties.
They are often very permanent and have very strong adhesion strength, and can be used both above and below the waterline.
3M 5200, a polyurethane sealant commonly found in marine chandleries, is basically for all intents and purposes a permanent adhesive and should not be used on boats for the most part. 3M 5200 has a bonding strength so high that it can often cause delamination or damage the gelcoat when you try to remove hardware bedded with it.
However, polyurethane sealants have some of the best materials compatibility, so the less aggressive ones, like 3M 4200 are very useful. SikaFlex 291 and 292 are probably better choices, but usually more difficult to find. For bedding ports, Sika 295 or 296 can be used in place of Dow's 795 Silicone.
Be aware that using a polyurethane sealant can make removing hardware much more difficult than using other sealants. There is a solvent, called DeBond 2000, which can be used to weaken the bond if you need to remove hardware that was bedded with 5200. One other issue with the polyurethane sealants is that they have a relatively short shelf-life, once opened. This is due to their being moisture curing compounds, and once exposed to the moist sea air... they start to cure...and you eventually end-up with a solid tube of cured sealant.
Polysulfide-based sealants are the best general purpose sealants for marine use.
They are not as aggressively adhesive as polyurethane-based sealants and generally a bit more elastic and flexible. They can be used both above and below the waterline, like the polyurethane sealants, and are better than polyurethane-based sealants for hardware that has to be re-bedded more frequently.
Polysulfide-based sealants are excellent for bedding wooden items, like rubrails and cockpit coamings, since it adheres fairly well to teak. The fact that these items often need to be removed for periodic re-finishing makes it ideal IMHO.
The main drawback of polysulfide-based sealants is that they tend to attack many common plastics—most commonly acrylic and polycarbonate. They are safe to use on acetal, delrin, nylon, and marelon fittings though. If you’re not sure what the fitting is made of, don’t use polysulfide-based sealants with plastic fittings.
Silicone-based sealants aren’t really sealants IMHO.
They’re really gasket materials, and need to have a minimum thickness and be kept under compression to work properly. Silicone-based sealants should only be used in above-the-waterline applications.
The only structural silicone sealant that I generally recommend is Dow 795. This is a structural adhesive which is generally recommended for bedding ports. It is not your common silicone caulk. However, beyond the very specific use of bedding acrylic* ports, it should not be used on boats.
Aside from bedding acrylic and polycarbonate ports, and certain plastic parts, like Beckson ports, and covering the exposed ends of cotter pins—it really has no place on a boat—primarily due to the residual silicone contaminants silicone can leave behind.
These contaminants are almost impossible to remove thoroughly, and will prevent other sealants and paints from adhering to the surface properly. Even strong adhesives, like epoxies, have trouble bonding if the surface has silicone contaminants on it.
One other use of silicone is for sealing potable water tanks. However, I highly recommend that you use only NSF approved silicone sealants for potable water tanks and systems. These will not have any toxic components, unlike some of the other marine-grade sealants which may contain isocyanates.
Some silicone sealants are acid-curing and should never be used on metal. These are generally easily detectable by the strong vinegar smell caused by the acetic acid that is contained in them.
The last sealant is butyl rubber glazing tape.
This may be one of the most versatile sealants you can use on a boat. However, it has the weakest adhesion and tensile strength and highest elasticity of any of the sealants. It is also the most easily affected by other chemicals, as it doesn’t cure like the other sealants do.
Unfortunately, many petrochemicals and common solvents will dissolve or damage it. Because of its sensitivity to petrochemicals, I generally don’t recommend it be used below the waterline.
It is great for bedding deck hardware, especially things like chainplates, where a certain degree of movement is unavoidable. It has the greatest materials compatibility of all the sealants and is also probably the least expensive of them.
One major advantage of butyl tape, since it doesn’t cure, is the working life. This makes it ideal for bedding things like traveler and genoa fairlead tracks. The lower physical strength of butyl tape generally isn’t an issue due to the large number of fasteners generally used on these tracks.
Two other sealants of note for marine use are 3M 4000 UV and BoatLife’s LifeSeal.
3M 4000 UV is a polyether based sealant, and as such is generally fairly well suited for use with plastics. However, it is not recommended for below-the-waterline uses. It cures relatively quickly, as it is tack-free in under half-an-hour and cures in 24 hours or so.
BoatLife’s LifeSeal is a hybrid polyurethane/silicone sealant. As such, it isn’t as permanent as a pure polyurethane sealant, but is more an adhesive bedding compound than a pure silicone material. However, it still has many of the contamination issues that any silicone based sealant will have. It also is fairly inelastic, and IMHO, 3M 4000 UV is a better choice.
*Most marine ports should be made of cast acrylic, rather than polycarbonate, due to several physical characteristics that make polycarbonate less suitable. First, polycarbonate tends to deform under load, which can cause it to shear the adhesion of the glazing to the underlying sealant. Second, it is less scratch and UV resistant than acrylic. Third, it is more difficult to find polycarbonate in UV/scratch resistant versions in sizes thicker than 1/4″.