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The Lowdown On Caulk

A stroll through the caulk and adhesive aisle of any marine chandlery will reveal a dizzying array of products. I recently walked through the ship’s store for a local boatyard I frequent, and a quick perusal of the stockroom reveals more than 16 different varieties of caulk in 10-ounce tubes (the long cylindrical cartridges are designed to be used in caulking guns), as well as a bevy of smaller 3-ounce squeeze tubes.

A sampling of these caulks includes fast and slow cure, permanent and removable, anti-mildew, polyurethane adhesive/sealant, UV stable, polysulfide bedding compound and silicone sealant, as well as glazing adhesive (for windows), teak deck caulk and adhesive, single and two-part assembly adhesive (used in a proprietary dual cartridge gun), and so on.

It is therefore no wonder that many boat owners and professionals alike have difficulty choosing the correct product and, most importantly, obtaining the best, most long-lasting results. Polyurethane adhesive/sealants and polysulfide bedding compounds (more on what this means in a moment) are, for the most part, designed for both above and below the waterline use, while silicones are generally not designed to be used below the waterline.


Most polyurethane sealants, unless specifically designated as such, lack much, if any, UV resistance, and as such their use above the waterline needs to take this into account. For example, laying wide, exposed beads or fillets with this material will often result in its deterioration. If it's shielded beneath hardware, however, it's fine.

Conversely, both silicone and polysulfide typically offer good resistance to the sun's rays, while polysulfide also possesses excellent resistance to petroleum-based products, including gasoline and diesel fuel.

Silicone sealants are very durable and last a long time; however, they are not appropriate for bedding objects such as through-hull fittings, struts, transducers, etc. Leave those jobs to polyurethane and polysulfide. While it’s often considered heresy to many in the marine industry, silicone may be used for bedding deck hardware, fasteners, etc. Why is its use frowned upon by many professionals?

Silicone in all its forms, be it sealant, wax, window/windshield coatings, lubricant, and even furniture polish, is the scourge of painters and varnishers everywhere. If you use silicone on or near a surface that will be painted or varnished in the near future (in some cases years, especially in the cabin), you run the risk of surface contamination.

The result will be the dreaded fish eye, an oval shaped area that will tenaciously and maddeningly repel coatings. In spite of this potential liability, however, if used carefully, there is a place for silicone sealants, especially high-quality varieties from well-respected manufacturers.

Several years ago I switched to SilPruf, a silicone sealant manufactured by General Electric, for bedding large, aluminum deck fixtures such as window frames and hatches. Obtaining an effective seal on these parts often proved challenging. However, SilPruf was equal to the job, and chronic leaks became a thing of the past. GE claims it has a weather-and UV-resistant lifespan of 20 years and I’ve had no reason to doubt it.


In order to make the proper caulk selection it is important to differentiate between the aforementioned adhesive and sealant. The former is, just as its name implies, a glue of sorts. It offers some structural support to the object to be installed or bedded. This adhesive ability is usually measured in tensile and/or peel strength. Sealants, sometimes called bedding compounds, on the other hand, do just that; seal or bed, but they offer little in the way of adhesion. Adhesive/sealants are multitaskers—they do both. The question you must ask yourself before choosing and using a caulk is, “Do I need to simply keep water out, or do I want to do that, as well as add strength?” There are times when an adhesive is a hindrance, particularly if it's likely that the component being bedded might need to be removed someday.

A good example of uses for tenacious adhesive/sealants (such as 3M's venerable 5200) are seacock and strut installations or hull to deck, cabin top, and keel to stub attachment. It's not likely that these items will be disassembled, and if so, it won't be for many years, hopefully.

We've all heard legendary stories about the aggressiveness of some adhesive sealants. Here's another, I once removed an 8,000-lb. lead ballast section from the keel stub of a 36-foot sailboat. Once all of the keel nuts were removed, the lead refused to separate from the fiberglass, even when suspending its full weight, thanks to the adhesive sealant that was used during the assembly process some 15 years earlier. You wouldn't want to use that type of material to install a light fixture or antenna base that you someday may have to remove.

Let's assume you are installing a radar scanner on your vessel’s hardtop or rebidding a port light in your cabin side. Proper bedding procedure calls for meticulous cleaning of the surfaces to be caulked or bedded, and use of denatured alcohol, mineral spirits, or 3M General Purpose Adhesive Remover to de-wax and de-oil the surface. A light scuffing of the surfaces, if they are very smooth gelcoat or metal, is helpful, providing some "tooth" to which the bedding can adhere (see PassageMaker issue June ‘09 Gearhead for more on bedding on cored fiberglass structures as well, as tips for bedding preparation).

If you are installing a new piece of hardware or re-bedding existing items, and the substrate is composite fiberglass, utilizing either plywood, balsa, or some type of synthetic material for the core, the proper approach involves ‘reefing and back-filling.’ After drilling the fastener hole, slightly oversized—approximately 25-percent—scrape out between 1/4 and 3/8 inch of core around each hole, while leaving the upper and lower fiberglass skin intact.

Fill this void with a mush of thickened epoxy, about the consistency of mayonnaise. Some boatyard pros tape the bottom of the hole and squirt a thinner epoxy mixture, more like honey, in with a syringe. This works well and has the added advantage of forming a better bond with the core material. It is, however, difficult to do on vertical surfaces. This process will prevent water intrusion into the core as well as preventing the fasteners from crushing the core material.

Once the epoxy hardens, re-drill the hole, include a chamfer around each hole to retain the bedding compound, clean the surface again, apply a liberal amount of the bedding compound and install the hardware. If the bedding is on deck and will be visible, be sure to select a variety that is both UV and mildew resistant.

Do not apply the bedding compound to interior portions of the hardware, washers or backing plates. Doing so will only retain water, leading to hardware corrosion and increasing the likelihood of water migration into cored structures. As counterintuitive as it sounds, it’s desirable to have water leak into the vessel when the exterior bedding has failed, it’s your signal to re-bed deck hardware. Bedding interior hardware only inhibits this signal.

When bedding, plenty of squeeze-out is OK—it lets you know you have used enough bedding material. In fact, you may opt to mask the area around the hardware footprint in order to minimize cleanup. Under no circumstances should you use any solvent not specifically approved by the sealant manufacturer for cleanup. In some cases, doing so may inhibit the curing process, leaving the sealant perpetually tacky.

The next step is where many bedding installations go wrong. Some dockside wisdom calls for not fully torqueing the fasteners until the bedding compound cures. In my experience this guidance is misplaced. Using this approach often leads to loose fasteners as the cured sealant “gasket” is crushed or squeezed out when the hardware is loaded, which in turn leads to leaks. Instead, go ahead and torque the fasteners, get the squeeze-out, clean up, and you are done.


Boat owners often ask, "When is it time to re-bed deck hardware?" Unfortunately, too few ask this question, waiting instead until a series of leaks develop. It is nearly impossible to answer this question because of the number of variables involved. These may include, the type of bedding compound that was initially used, if the factory followed good bedding etiquette, if there was too much or too little squeeze-out, if the cored holes were epoxy filled (if not, as mentioned above, the core may be crushed, releasing tension on the fasteners, which leads to leaks as well as a wet core), and so on.

Additionally, the climate in which the boat lives and the color of the hull and deck play a significant role in bedding longevity; in my experience dark-hulled boats tend to be harder on bedding and thus leak more than their lighter colored brethren. Think about it. How many of us have walked barefoot across a blacktop parking lot when returning to the car from a day at the beach? I learned at an early age to walk on the white lines that marked the parking spaces. Boats are no different, in measurements I've made, on a 90-degrees-Fahrenheit day. A light-colored hull or deck, in direct sunlight, may read 90 to 100 degrees Fahrenheit when using an infrared pyrometer, while black, blue and green hulls will easily reach 150 to 180 degrees. Dark colors also hasten post-curing, but that's a topic for a future paint and fiberglass column.

The expansion and contraction that results in such wide temperature swings, from day to night, cause substantial expansion and contraction, leading to hardware bedding failures. Generally speaking, I expect bedding to last between seven to ten years, and in some cases 15 years, with the spread between these figures heavily influenced on the aforementioned criteria of quality of material and application methods.