Understanding Sealants: We Test Urethanes, Sulfides & Silicones

In our tests of 11 one-part sealants and adhesives, we found many products that will do some jobs well, but few that do everything.


Choosing the right sealant or flexibile adhesive for the job used to be fairly straightforward. Brands were few and products were fewer, and there was usually somebody around who could tell you which compound was best for bedding cleats or sealing joints.

Thats no longer the case. There are many more brands available today, with old favorites such as BoatLife and Sikaflex offering variations on their old, familiar products. Many of these have been formulated to do specific jobs. Thats good news for the do-it-yourselfer, but with variation comes an increased likelihood of making a mistake.

For this review, we looked at 11 products from three manufacturers (Sikaflex, 3M, and BoatLife), including some old standbys and some newer variations. We checked their handling characteristics, and tested each for bonding and adhesive strength.

Many of these compounds come in single- or two-part (mix and apply) formats. Both have similar strength, so we looked at just the single-part products for the sake of convenience. Single-component sealants and adhesives are much easier to use than those that require mixing two or more components. One-part products also avoid the problems associated with limited pot-life.

We can say that your choice of product depends on what you are doing; in other words, there is no one product best suited for all materials and applications. Unfortunately, manufacturers don’t always do a good job of making applications clear. For that reason, we recommend reading labels and instructions carefully, and even contacting the manufacturer for more detailed information when you feel it necessary.

All the products we looked at are synthetics, which have largely displaced the old natural materials for sealing and bonding chores where water- or air-tight sealing is needed with some measure of give or flexibility. Other common uses include joining materials together, often in conjunction with mechanical fasteners for added adhesive strength and sealing. Lastly, these materials are used to isolate one surface from another to prevent noise, corrosion, etc.

Silicones have been around for many years. They are elastic and chemical-resistant, and are very effective for sealing and insulation. They have additional capabilities now, and some variants can be sanded and used as deck sealants.

The next category is the polysulfides, which also have been in use for a long time and are being displaced.

The third category, polyurethanes, are the materials of choice for applications where high adhesive strength for permanent bonding is desired, such as in hull and deck joints. However, the polyurethanes have also expanded their realm of usability, especially those from Sikaflex.

Sealants and adhesives have two basic properties to consider. The first is cohesion, which is the resistance of the cured product from being torn apart from itself when put under mechanical stress. The second property is adhesion, which is the tendency of the sealant to bond to the substrate (wood, fiberglass, etc.). In our Value Guide (pages 6-7), tensile strength is a general approximation of adhesive properties, but remember that adhesive properties are influenced by several factors. The first is the proper preparation of the surface. A slightly roughened surface provides more surface area at the microscopic level-so-called tooth-for an adhesive to mechanically bond to.

Other considerations include the porosity of the bonding surface to allow diffusion below the surface, and the chemical characteristics of the adhesive and substrate that may allow molecular substrate/adhesive mixing during cure.

What We Tested
Polyurethanes. These are among the newest and most versatile adhesive/sealants, with moderate to extremely high adhesive qualities. One-part types have cure rates from 24 hours to several days. Unlike two-part systems, one-part polyurethanes rely on an external atmospheric catalyst such as moisture, oxygen or even UV light. The water catalyst is by far the most common, meaning that it needs atmospheric moisture to cure. Applying the product too thick can keep moisture out, preventing a full cure and dramatically reducing adhesion.

While many polyurethanes can be painted and some sanded, it usually is a good idea to let the product dry before paint application. Remember, painting over something that has stretch/flex characteristics is a tough chore for most paints.

On the other hand, polyurethanes tend to be somewhat light-sensitive, so those installations where there will be constant light exposure will definitely benefit from paint, or the careful selection of a UV-resistant variant.

Polysulfides work above and below the waterline, and especially well for underwater wooden boat seams. Polyurethanes, which have similar applications, would not be our first choice for this use.

Both types of product can be applied on damp surfaces (since they need moisture to cure), although this is not a license to skip proper surface preparation-clean, dry surfaces are the recommended optimal surface. One objection to the polysulfides has been the long cure time-up to seven days or more depending on bead thick ness. (Two-part systems can speed up cure times and also have good self-leveling characteristics, which can be helpful in sealing decks.) The polysulfides are quite UV-resistant, and very resistant to fuel and oil. In fact, some variants of the polysulfides are used in the aviation industry for sealing fuel tanks. Another objection to polysulfides is the unpleasant smell.

However, regardless of the product you use, good ventilation is a must, smells notwithstanding. In fact a respirator may be required in some inside situations, particularly with two-part systems.

Silicones. There are two basic types of marine silicones. One, based on acetic acid, has the familiar smell of vinegar, while the other is based on ammonia and has no smell. Since the acetic acid variety is corrosive to metals, you should stick with the ammonia-based version. Also, don’t use non-marine silicones, as they can have a number of additives that are not conducive to marine uses.

Silicones have extreme temperature tolerances (up to 500F or more) while still remaining flexibility. Thats why silicones have largely displaced preformed gaskets in the motor assembly process. (You have to be careful here, as only certain approved silicones do not contaminate oxygen sensors). Their cohesive and adhesive strength is lower than the other categories, but this may be exactly what is desired. Silicones work especially well in sealing plastics and glass and have excellent UV characteristics. Silicones also have a longer shelf life, both opened and unopened, than the products which cure from atmospheric moisture. One last caution. Be very careful to not apply silicones around active painting processes where final finish must be perfect, as the solvents in the curing silicones can cause fisheyes in drying paint.

How We Tested
To test our adhesive/sealants we developed our own simplified versions of industry standard lap shear and peel tests. In the lap shear test, two overlapping pieces of material were bonded together over a 1 square inch surface area with the same very light clamping force used on each sample to generate approximately the same thickness in the cured bond. The pieces were then pulled apart with a 1500-pound pull hand winch connected to an appropriately rated pull-scale to gauge the break point in pounds. Because we were unable to have perfect longitudinal alignment, our results were lower than the manufacturers, but consistent.

In the peel test, the same size samples were used, but this time the force was from a 90 angle with a dial indicator inserted to accurately measure the break point (elongation factor). This was done using unfinished, dry wood-to-wood, clad aluminum-to-wood, and clad aluminum-to-fiberglass test samples. All test materials were cleaned according to manufacturer instructions. And all the tests were done at least twice and consistent results averaged together. The chart notes whether the sample failed at the wood, aluminum or fiberglass surface, or whether there was a cohesive failure (with the sealant pulling apart, leaving remains on both samples). We even experienced some substrate failures (so noted) with the wood partially failing as well when the high-strength polyurethane adhesives were used. The stretch-before-give numbers were not as consistent as we expected, perhaps reflecting more give in some products, and not causing a substrate failure at high-stretch values. We also noted the 3M products were not as effective on aluminum, but outstanding on wood and fiberglass (FRP).

Generally speaking, products that show high tensile strengths don’t do well in peel strength tests. They tend to be more rigid, and don’t distribute peel loads as well as do more-flexible products.

3M Products
5200/5200 Fast Cure. 3M 5200 polyurethane is a strong adhesive (with the highest tensile strength of the products tested) that also provides sealant properties, and is useable both above and below the waterline. It is also paintable as are most polyurethanes. It does take several days to cure, and is best used for permanent adhesion of wood and fiberglass.

5200 Fast Cure is a newer version, which cures in 24 hours or less, while retaining better than 90% of the adhesive strength of 5200. Working life is short-about one day after a tube is opened-and it comes only in white. Both 5200 products come in smaller tubes that seem to keep the compound fresher longer, and also avoid waste.

4200 Fast. This is another new polyurethane which has lower (but still substantial) adhesive properties than 5200 (boatbuilders have told us that 5200 is too strong and permanent for many applications). This new compound is designed for bedding and sealing chores where future removal may be needed, such as with deck hardware. In our tests, it proved to be stronger than its stated specs, but not as sag resistant as, for example, the Sikaflex products.

101. This polysulfide is slow drying (at least two days), paintable, and very UV- and chemical-resistant. Like most polysulfides it has an offensive rotten-egg odor. Its also thick and gooey (like roof tar) and doesn’t have a particularly smooth appearance. It is well-suited for sealing hull seams on wooden boats.

BoatLife Products
Life Seal. This is a polyurethane/silicone combination product with exceptional versatility and good adhesive properties. In addition to wood, metal and fiberglass, it can be used as an adhesive on many plastics, where most polyurethanes and polysulfides do not do well. It may be used above or below the waterline (except on wooden boat seams), and has the lowest application temperature of all tested products at 10F. It is not paintable, but otherwise is the most versatilel of the products we looked at.

Life-Calk. A slow drying polysulfide, Life-Calk, similar to 3M 101, may be best suited for sealing wooden hull seams. It is paintable and may be applied underwater in an emergency. It is not a strong adhesive. A 10.6 oz. cartridge is $8.65. A thinner version is called Liquid Life-Calk (the consistency of ketchup). We found that kneading the tube was necessary to properly mix the compound. It took more than seven days to begin curing, compared to two for the 3M 101, a better choice, we think.

Marine Silicone Rubber. This odorless product is very flexible and particularly well suited for sealing glass and plastics. It is chemically resistant, but not a strong adhesive (although it tested stronger than its specs), nor suitable for constant underwater immersion.

Sandable Silicone. One of the few silicones that can be painted, Sandable Silicone is slower-drying than the Marine Silicone and is designed primarily to be used as a simple, alternative teak deck joint sealer instead of the more common (and complex) two-part polysulfide and polyurethane products designed for teak deck sealing. It, too, performed slightly better than its specifications.

Sikaflex Products
298 (formerly 231). This polyurethane adhesive is best used for sealing and bedding, much like 3M 4200. It has greater thickness than other brands and tends not to run or sag. It does have a longer cure time. Special pretreatments are available for all Sikaflex polyurethane products to further enhance their adhesive qualities. In our tests, 298, like all the Sika products, performed up to spec, without the need for primers.

291 (formerly 241). 291 and the slow-curing version 240 (suitable for use in potable water systems), are Sikas general purpose adhesive/sealants, which bond well to a large variety of substrates (except plastic or glass), using pretreatments where necessary on more difficult materials. They are also a bit thicker than other brands for good sag resistance.

292. A new high-strength polyurethane adhesive, this is Sikas answer to 3M 5200. Sikaflex 292 is more effective on metals, such as aluminum, and dries quicker than even 5200 Fast. It is not, however, for underwater use.

The Sikaflex products, popular with professional boatbuilders, did not come with any directions on the cartridges we tested. The company told us that it would send detailed technical data sheets to anyone who would like them. A spokesman subsequently informed us that labels would be redone to provide users with more information.

Generally you will not see too many Sikaflex products other than 298 (231) and 291 (241) advertised in the big marine catalogs, but you can order them through marine catalog houses. Sika has an extensive array of pretreatment chemicals, which may be needed for some jobs/materials, and also a one-part polyurethane deck sealant, 290DC, that we did not test.

There is no shortage of choices, but if you ask yourself a few questions, you can easily narrow your options.

Cost should be the least of your concerns, unless you are going to be using gallons of the stuff. Performance is paramount.

First, will the product be used under water? That puts you in the realm of polyurethanes and polysulfides. Do you want a one-part product that has quick cure time? That narrows your options to the polyurethanes.

As a result of our testing, we found some favorites based on ease of use and versatility. BoatLife Life Seal was a top all-around performer, even working well with most plastics. Only its inability to be painted could be considered a drawback. And it is primarily a sealant, with adhesive qualities less than the high-strength products, but still good.

As a group, the polyurethanes (particularly the faster curing versions) were our favorites over the polysulfides for adhesive or underwater chores. 3M 5200 and 5200 Fast Cure were strong adhesives for fiberglass and wood (but not metal) both above and below the waterline. Sikaflex adhesives were the most effective products when bonding to aluminum. The only areas where polysulfides may excel over the polyurethanes is where fuel and solvent resistance is needed, and possibly for underwater wooden boat seams.

Silicones are now available in a number of variations, and are particularly effective as sealants where UV, high temperature, and chemical exposure are a problem. Stick with the odorless, ammonia-based types specifically designated for marine uses.

Finally, surface cleaning and preparation will make or break the use of these products. We experimented by leaving just a little surface dirt on a substrate, and learned that no product works nearly as well as with a clean, grease-free surface.

Contacts- BoatLife Products, Life Industries, 2081 Bridgeview Dr., N., Charleston, S.C. 29405; 803/566-1225. Sikaflex Corporation, 30800 Stephenson Hwy., Madison Heights, MI 48071; 248/577-0020. 3M Marine Trades, 3M Center Building 223-6S-06, St. Paul, MN 55144; 612/733-1110.

Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 45 years. Supported entirely by subscribers, Practical Sailor accepts no advertising. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida. You can reach him by email at practicalsailor@belvoir.com.


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