A quick walk through the caulk and adhesive aisle of any marine chandlery will reveal a dizzying array of products. Here at the boatyard, a quick perusal of our own small stockroom reveals over 12 different varieties of caulk in 10-oz. tubes (the long cylindrical cartridges designed to be used in caulking guns), as well as a bevy of smaller 3-oz. squeeze tubes.
A sampling of these caulks includes fast and slow cure, permanent and removable, polyurethane adhesive/sealant, 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), etc, etc.
It's no wonder that folks have a hard time choosing the correct product and getting good results from it. So, how do you decide what to use? Fortunately, a few simple guidelines can be followed to aid in proper selection.
Polyurethane adhesive/sealants (more on just what this means in a moment) and polysulfide bedding compounds are, for the most part, designed for both above and below the waterline use. Silicones are not designed to be used below the waterline. Most polyurethanes, unless specifically designated as such, do not have very good UV resistance, so take this into account when using above the waterline. Don't lay wide, exposed' beads or fillets with this material. If it's shielded beneath hardware, however, it's fine. Both silicones and polysulfides have good resistance to the sun's rays. Interestingly, polysulfides also possess excellent resistance to petroleum products.
Silicone sealants are very durable and long-lived; however, they are not appropriate for bedding objects such as through hull fittings, struts, transducers, etc. Leave those jobs to the polyurethanes and polysulfides. Silicone may be used for bedding hardware, fasteners etc. Keep in mind, however, silicone in all its forms-be it sealant, miracle wax or lubricant - is the scourge of painters and varnishers. If you use silicone on or near a surface that may be painted or varnished (in some cases years later, especially in the cabin), you run the risk of contaminating that surface. The result will be the dreaded fisheye, an oval-shaped area that will tenaciously repel all paint and varnish.
Be mindful, when purchasing silicone sealant, of the two different varieties. One is based on acetic acid, hence the vinegar odor, while the other is ammonia-based. The latter has little or no smell. The acetic acid version can be corrosive to metal, particularly aluminum. Err on the side of caution; buy only the ammonia-based sealant and you won't have to worry about using it in the wrong application.
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, can seal or bed as advertised without offering much adhesion. Adhesive/sealants are multitaskers; they do both. For example, Sikaflex 231 (Sika Products, Madison Heights, MI, www.sikasolutions.com) bedding compound has a tensile strength of 125 psi, while Sika 291 adhesive/bedding compound offers 225 psi of adhesion.
The question you must ask yourself before choosing a caulk is, do I simply need to keep water out, or do I also need to add strength? There are times when an adhesive is a hindrance, particularly if it's likely that the component being bedded might be removed someday.
A good example of uses for tenacious adhesive/sealants (such as 3M's 5200, 3M Marine, St. Paul, MN, 651-773-1740, www.3m.com) 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 isn't likely to be for many years.
We've all heard legendary stories about the aggressiveness of some adhesive/sealants. Here's another: Our yard recently removed an 8,000-pound lead ballast section from the keel stub of a 36' sailboat. Once all of the keel nuts were removed, the lead refused to separate from the fiberglass, even when suspending its full weight. The 5200 adhesive/sealant did an excellent job of adhering the lead to the fiberglass. You wouldn't want to use that type of material to install a dorade, navagation instrument or antenna base that you someday may have to remove.
Let's assume you are installing a radar scanner on your trawler's hardtop or a port light in your sloop's cabin side. Proper bedding procedure calls for meticulous cleaning of the surfaces to be caulked or bedded. Use denatured alcohol or 3M General Purpose Adhesive Remover to de-wax and de-oil the surfaces. A light scuffing of the surfaces, if either is very smooth gel coat or metal, is helpful, providing some "tooth" to which the bedding can adhere.
If you are installing a new piece of hardware (this actually applies to rebedding existing items as well), and the fiberglass is cored with plywood, balsa, or some type of synthetic for the sandwiched material, the proper approach is to reef and fill. After drilling the fastener hole, the outside hole should be slightly oversized-approximately 25%; scrape out between 1/4" - 3/8" 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, which is a nightmare. It also prevents the fasteners from crushing the core material when they are torqued.
Once the epoxy hardens, re-drill the hole, the same size as the fastener this time (not oversized) and bevel the edges slightly. The beveling will hold caulk in an 0 ring-like clean configuration. Clean and de-wax the surface again (after it has cured, epoxy will leave behind a wax called amine blush), then apply a liberal amount of bedding compound and install the hardware.
Plenty of squeeze-out is OK, it lets you know you've used enough bedding material. In fact, you may opt to mask the area around the hardware footprint in order to minimize cleanup.
The next step is where many bedding installations go wrong. Resist the temptation to fully torque down all the fasteners, clean up, and walk away. Instead, firmly snug each fastener and then wait until the bedding compound cures before doing the final torque. Curing may take days, depending on the material, air temperature humidity-most urethane sealants actually cure in the presence of moisture, which is why they're excellent for emergencies underwater repairs. Now, instead of squeezing all of the sealant out of the interface, you've left some behind, creating a custom-made gasket.
Customers often ask, "When is it time to rebed deck hardware?" It is nearly impossible to answer this question because of the number of variables involved; What type of bedding compound was used initially? Did the factory follow good bedding etiquette? Was there too little squeezeout? Are the cored holes epoxy filled (if not, as mentioned above, the core may be crushed, releasing tension on the fasteners, which leads to leaks as well as wet core), and so on?
Additionally, the climate in which the boat and lives and the color of the hull and deck play a significant role in bedding longevity. My experience has been that 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? 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 here in the yard, on a 90-degree July day, a light colored hull or deck, in direct sunlight, may read 90 to 100 degrees on the 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 article.
Wide temperature swings, such as those that occur from day to night, cause substantial expansion and contraction, thus leading to hardware bedding failures. The boatyard rule of thumb is, however, after ten years, the bedding owes you nothing. If you get 15 years out of it, without an% leaks, you are doing very well.
Masking tape is most often used in paint applications, however, don't overlook its usefulness when bedding. If you intend to bed a piece of deck hardware, especially when non-skid material is used, masking can reduce clean-up time considerably.
Steve C. D Antonio is the manager of Zimmerman Marine in Mathews, Virginia. This taken from Seaworthy April 2003.
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