Synthetic Rubber-Based Adhesive Sealants
From "The Practical Sailor Library - Volume IV - Do-It-Yourself Improvement Projects"
Click here for a helpful summary table of Adhesive Sealants

Years ago, no matter what sort of a seam-sealing or bedding operation you had in mind, you picked up a putty knife and a can of oil-based bedding compound. Oil-based compounds rarely failed due to problems caused by incompatibility with other boatbuilding materials; they always failed within a few years, however, due to drying, cracking, and lack of adhesion.  Over the past decade, oil-based bedding compounds have all but disappeared from the shelves of marine hard ware stores and ship chandleries. They have been supplanted by synthetic rubber adhesive sealants. The synthetic rubber compounds typically last for ten to twenty years or more. Nevertheless, a tube of adhesive sealant must be chosen much more carefully than a can of bedding compound; the various polymers exhibit vastly different properties, and frequently fail due to incompatibility with other materials.

When we first began to examine these newer polymers, we had hoped to identify a single compound that would be suitable for all bedding and sealing uses, both above and below the waterline. What we discovered was that even though all types of sealants are described with words such as "waterproof," "permanent," "flexible," and "safe for use above and below the waterline"; they are not identical or interchangeable. The use of each type of polymer is, in at least one sense, a compromise, and each has advantages offset by disadvantages.

Although there are various proprietary compounds (often mixtures of two or more polymers) that exhibit various combi-nations of characteristics, the choice of an adhesive sealant usually comes down to one of "the big three": the silicones, the polysulfides, and the polynrethanes. Its no accident that these are the most common and most widely recommended marine adhesive sealants; with one of them, virtually any bedding and sealing problem can be solved.

Of the three major categories of synthetic rubbers, all have applications aboard boats built of metal and wood, as well as those built of fiberglass. We must admit, however, that it is easier to choose a compound for use on a fiberglass boat, simply because there are fewer variables associated with fiberglass surfaces. On wood and metal boats, surface preparation plays a much larger role in the performance of an adhesive sealant. There are also variables introduced by questions of compatibility with paints, sealers, and preservatives that have been used previously, or may be used subsequently. These questions make it more difficult to define rules of thumb.

For most of us, the choice of an adhesive sealant is almost an afterthought. On the way to the cash register with our latest nautical acquisition, we stop long enough to pick up a tube of something to bed it in-something that seems to have all the right "key words" printed on the container. Beginning with the silicones, we will attempt to determine what key words to look for, and we will try to establish some ground rules for choosing a bedding or sealing compound. Armed with a working knowledge of the advantages and disadvantages of each type of polymer, it is not too difficult to narrow down the choices, and to choose the best compromise.

The Silicones
RTV (room temperature vulcanizing) silicone synthetic rubber is an elastomer derived from the silicon metal found in sand. Various curing agents give silicone its characteristic smell of vinegar or ammonia, but most marine silicone sealants are of the "acetoxy-cure" variety, which give off acetic acid as they cure. These are based on technology developed in the late 1950s by a French company, Rhone-Poulenc. Regardless of the curing system, all silicones cure by reacting with moisture in the air.

Silicone caulks are commonly found in the neighborhood hardware store (where they will normally be priced lower than in marine outlets), so the temptation exists to replace the "boat" silicones with "household" lines. For belowdecks applications, the substitution might be acceptable, but the marine distributors insist that their formulations will prove to be a better long-term value. They tell us that the household silicones contain a fungicide (anti-mildew agent) that rapidly turns yellow when exposed to ultraviolet rays. The marine silicones also contain a fungicide, but employ UV-screening agents to prevent yellowing.

Perhaps the single greatest advantage of silicone adhesive sealant and the reason for its popularity in marine applications, is its fast cure time. Most silicones will "skin" (lose tackiness) in half an hour or less, and cure completely in 24 hours. Thus, a fitting can be bedded in silicone one day, and put into service the next. Silicone sealant is virtually non-shrinking, highly resilient, and retains its elasticity throughout a wide temperature range. Silicone sealants have a long service life; twenty years is the figure most often given by manufacturers.

These desirable qualities have made the silicone compounds the nearly unanimous choice for bedding owner-installed add-on equipment, and for bedding and sealing applications on most production boats. Unfortunately, however, the silicones have some serious drawbacks as well. The silicones have the lowest adhesion coefficient of any of the three major categories of marine synthetic caulks. This makes them suspect when applied in the form of bead of caulk in working seams, or in applications where it is necessary to fill a large void.

While of low adhesion, the silicones exhibit high cohesion. This means that once cured, they form a strong, plastic mass. The qualities of good cohesion and poor adhesion are demonstrated by the fact that silicone caulk can be pulled out of a seam in one long "rope." This quality makes silicone compounds most suitable for use as a formed-in-place "gasket" under compression. A good example of this is in the case of bedding for a stanchion base, where the compound is held in place by the compression of the mechanical fasteners.

Another notable drawback of the silicones is their absolute refusal to hold paint. Many of the silicone suppliers do not state this very dearly on their containers and some frustrating situ-ations have been created. We have heard of numerous instances where boatowners have missed or misunderstood the warning to "paint before applying" and splashed silicones around care-lessly because they planned to paint afterward. Their biggest shock came, after laboriously removing the excess, to find that an oily residue from the silicone caulk had soaked into the fiber-glass, rendering it almost permanently unpaintable. There are cleaners used in the auto-painting business to remove silicone waxes prior to painting automobiles that will help (DuPont's Prep Sol, and Martin Senour's Kleanz Easy), but the silicone residue can continue to leach out of a wood or fiberglass surface for years. The use of silicone waxes and sealants is one reason why an epoxy barrier coat or an epoxy primer is almost universally recommended under polyurethane paints on fiberglass hulls and decks.

On wooden boats, where painting falls under the heading of "routine maintenance," it would be wise to avoid the use of products containing silicone altogether. If you use silicone caulk on a fiberglass boat, complete any painting that may be necessary before using silicone. Be careful about excess, and wipe it up immediately with a dry cloth or paper towel. Don't allow the excess to set, with the intention of trimming it off later. This allows more time for the silicone to invade the fiberglass, jeopardizing future paint adherence.

Paint is not the only material that does not adhere well to silicone; silicone does not adhere well to silicone. A damaged seam sealed with polysulfide or polyurethane, for example, can be repaired with the same material once the damaged material has been sanded or cut away. On the contrary, when replacing a fitting bedded in silicone, all of the old material should be removed, the mating surfaces cleaned, and the part should be completely rebedded in fresh material. A "smear-around-the-edges" solution to a leak is an unseamanlike procedure at best. With silicone sealants, it is likely to be a short-lived solution.

It is easy to be confused by the fact that most manufacturers' literature will advise against the use of silicone sealants below the waterline in one paragraph, and recommend silicone for bedding plastic through-hull fittings in another. The prohibition against the use of silicone underwater is designed to discourage its use as a seam-sealer due to its poor adhesion (which gets noticeably poorer after prolonged continuous exposure to water). To avoid problems, most manufacturers would prefer that customers purchase their polyurethane and polysulfide sealants for use underwater, but these compounds are not compatible with all plastics. Since poor adhesion is not a problem when silicone is used in compression under a mechanicafly fastened through-hull fitting, silicone becomes the recommended sealant for these fittings by default.

Before it begins to set up, silicone sealants can be tooled effectively by smoothing with a finger and wiped up with a dry cloth. Paint thinner or mineral spirits is helpful for clean-up once the material begins to cure. Once fully cured, soaking in mineral spirits will help to soften the compound for mechanical removal by cutting, scraping, and rubbing with a course cloth.

Silicones come in "clear" and a variety of colors. They are recommended for formed-in-place engine gaskets that must withstand high temperatures; for bedding deck fittings that may have to be removed at a later date; for bedding securely fastened underwater fittings of polycarbonate plastics (ABS, Lexan, PVC, acrylic) that would react unfavorably with other adhesive seal-ants; for potting electrical junctions (for insulation and corrosion prevention); and for sealing narrow, mechanically fastened joints in cabin joinery.

Silicones are not sandable, and should not be used where painting is required. They are susceptible to a naturally occurring bacteria (bacillus niger) which can cause discoloration and deterioration. They are not highly resistant to solvent contact on prolonged exposure, and are not recommended for filling large voids, or for working seams.

The Polysulfides
Polysulfide bedding and bonding materials have been in use since World War II when they were developed by Morton Thiokol for the manufacture of "leak-proof, explosion-proof" aircraft fuel tanks. During the war, aircraft crews learned that "Thiokol" was ideal for quickly patching bullet holes in planes before sending them back into the air. Polysulfide adhesive sealant is still often referred tons "Thiokol," and the polysulfide polymer in all brands of polysulfide adhesive sealant is still manufactured by Morton Thiokol, Inc.

The first widespread use of polysulfides in boatbuilding was as a bonding and sealing agent between the planks in lapstrake wooden boats in the late 1950s. Remarkably, one large manufacturer, in order to test the effectiveness of the Thiokols, built a 26-foot lapstrake boat, removed all the mechanical fasteners in the hull, and ran the boat through an entire season of hard use with no separation or leaks developing.

Many experts feel that due to its combination of high bond-strength and high elasticity, polysulfide is superior to epoxy for the bedding of teak-strip decks laid over steel, plywood, or fiberglass, regardless of whether or not mechanical fasteners are used. In any event, polysulfide is the nearly unanimous choice for the seams in both laid and overlaid decks, due to its resistance to fuels and solvents in general, and to teak cleaners in particular.

Polysulfide adhesive sealants are available in both one- and two-part formulations, but boatowners seldom run across the two-part type; it is more commonly marketed to boatbuilders and boatyards. Polysulfide is available in various consistencies; from "pourable" (the thickness of honey), to "gun-grade" (applied through a caulking cartridge); to a thick, "knife-grade" (applied with a putty knife). While somewhat less elastic than the silicones, the polysulfides retain their flexibility throughout their useful life of up to about twenty years. This quality makes these materials ideal for working seams. Their strong adhesion, plus elasticity, makes for truly permanent joints.

The polysulfides cure slowly compared with other marine adhesive sealants, with the one-part formulations generally taking significantly longer than the two-part compounds. Thiokol-based sealants cure by exposure to moisture, and thus set more quickly in high relative humidity. The curing time is also affected by ambient temperature (the warmer the temperature, the faster the cure). Although the compound should be applied to a dry surface, polysulfides will continue to cure underwater, and boats can be launched with mechanically fastened fittings bedded in partially cured material. In an emergency, polysulfides can even be applied underwater, but this should be consid-ered a temporary fix rather than a permanent solution.

Tack-free times for one-part polysulfide reported by the manufacturers vary from as low as 30 minutes to as high as 48-72 hours; reported cure times vary upwards from 2-3 days to 7-
10 days. Of course, "tack-free" involves a somewhat subjective judgement; and since the material is inherently flexible, it is often difficult to feel the material and decide whether it is "fully cured" or just "nearly cured."

Unlike the silicones, the polysulfides are sandable, although to avoid breaking the bond with the substrate, it is important that the material be fully cured before sanding. In cases where sanding is anticipated, it may be helpful to prepare a test panel at the time of caulking to use in determining if the material is fully cured and ready for sanding. In most bedding applications, however, no sanding will be required, as the uncured material can be effectively "tooled" with a rag or putty knife wet with mineral spirits. Most manufacturers recommend that wooden parts be masked with tape if the excess material is to be tooled rather than sanded. This helps to prevent the compound from becoming lodged in the surface grain of the wood.

Also in contrast to silicone sealants, polysulfides are readily paintable with no special priming required. Manufacturers' instruction differ, however, on whether the material can be painted at the "tack-free" stage, or if it should be fully cured before painting. The "primers" for polysulfide that are occasionally seen on the shelves of chandleries, incidentally, are not for the priming of polysulfide sealants prior to painting; they are for priming the surface of oily woods prior to applying the polysulfide. Trim made of teak (the archetypal "oily wood") need not be primed when it is bedded in polysulfide and mechanically fastened to a hull or deck. (In this case, the sealant is functioning simply as a gasket.) When applied as bead of caulk in a hull or deck seam, however, the adhesion of any sealant is critical, and priming is an important step. It should be noted that seams may be “oily" and thus require priming because of the nature of the material, as in the case of teak. Or the wood may be oily because of its exposure to other materials, as in the case of seams in the bilge of a wooden boat which have soaked up oily bilge water or previous)y used oil-based seam compounds.

The clean-up of uncured polysulfide can be accomplished easily with mineral spirits or naptha (lighter fluid) Once the material begins to set up, stronger commercial solvents such as xylol, toluene, or methyl ethyl ketone may be required. Once polysulfide is fully cured, mechanical removal is usually necessary, by cutting, scraping or sanding.

Polysulfides will adhere to metal, glass, fiberglass, wood, or any combination of these. They should not be used for cementing PVC, acrylic (Plexiglas), ABS, or Lexan plastics, because the solvents in polysulfides can leach the plasticizer from these plastics and cause them to harden and crack. Silicone sealants are usually recommended by the manufacturers of products made from these plastics. The higher-quality plastic fittings made from Delrin, nylon, glass-reinforced nylon (Marelon), or glass-filled epoxy are not affected by polysulfide sealants.

The polysulfides are not resistant to high temperatures, but are all but impervious to fuel and solvent deterioration. This makes them suitable for formed-in-place gaskets for refrigeration equipment, fuel tanks, and fuel systems. Polysulfides are particularly suitable for bedding teak, and for bedding fittings around areas of possible fuel spills. They are an excellent bed-ding material for underwater fittings, with the exception of the plastics noted previously. Although their adhesion is not as great as that of the polyurethanes, this is a plus, when used on fittings that might have to be removed at some time in the future.

The Polyurethanes
The polyurethanes are adhesive sealants with the accent on the word adhesive. Although they are highly effective as waterproof and weatherproof sealants, polymer chemists routinely refer to urethanes as "adhesives" rather than sealants or bedding compounds -- in much the same way that they refer to the epoxies. In fact, the definition, "tough, flexible, waterproof, gap-filling, permanent adhesive," could apply equally well to either epoxy or polyurethane.

The owner of a fiberglass boat who does his own maintenance will find few, if any, applications for polyurethane com-pounds that cannot be handled effectively with polysulfides and silicones. For boat building and extensive modification projects, however, polyurethanes are "state-of-the-art" for applications that call for a permanent, self-adhesive, waterproof gasket. The common examples are the installation of chainplates, ballast keels, centerboard trunks, and hull-to-deck joints.

This is not to suggest that polyurethanes are for use by professionals only, and should not be used by the boatowner. But the boatowner who chooses to use the material should do so with the knowledge that parts bedded in polyurethane are not just difficult to remove, they can be nearly impossible to remove without damage to the part, the substrate, or both.

The polyurethanes, like the polysulfides, are available in both one- and two-part formulations, but the one-part formula-tion is far more commonly available in boatyards and marine outlets. Also like polysulfides, most urethanes are moisture-cured, but at least one (PRC's PR-5365-M) cures by reacting with oxygen in the air. Cure times for polyurethanes are generally longer than the silicones and shorter than the polysulfides. Polyurethanes will cure in a broad range of temperatures, but like the polysulfides, the actual cure time is a function of temperature, humidity, and the volume or surface area of the adhesive. Tack-free times at 75 degrees F and 50-percent relative humidity given by the manufacturers range from 30 minutes for Sika's Sikaflex 241, to as much as 48 hours for 3M Company's Marine 5200 Adhesive/Sealant. Three days is given as the cure time for most urethanes, with 5200 the slowest at 7 days.

Another notable similarity between the polysulfides and the polyurethanes is that they can both be sanded and painted. Some manufacturers recommend wet sanding of cured urethane, which serves as a tip-off that the urethanes do not sand easily. The flexibility of the two materials is also similar, but cured polyurethane is generally not as resilient as polysulfide. Polysulfides exhibit a fairly consistent degree of "rubberiness," and are markedly less flexible than the silicones. The flexibility of the polyurethanes, while similar to that of the polysulfides, seems to vary more from one brand to another. They range from "about as flexible" to "decidedly less flexible" than the polysulfides.

The prohibition against the use of polyurethanes for bedding thermoplastic fittings is the same as for polysulfides, but interestingly, the problem is exactly the opposite. Solvents, leaching from some plastics, can react with polyurethanes and cause a failure of the adhesive sealant compound. The end result, of course, is the same: a failed bond and hard-to-pin-down leaks.

A final area of similarity between the polysulfides and the polyurethanes is in the tooling and clean-up procedures. Before polyurethane begins to skin over, it can be tooled and removed with a rag wet with mineral spirits. The mineral spirits does not seem to dissolve the compound, but it does seem to lubricate the surface of the tool, or the area surrounding a bedded fitting enough so that the rag can "push" the excess away. Once the material begins to set up, the same industrial solvents recom-mended for polysulfide (xylol; toluene; methyl ethyl ketone; 1,1,1 -trichtoroethane) are required to remove it; once fully cured, these solvents will be of some help in softening the compound for mechanical removal.

A major difference between polyurethane and polysulfide centers around their use as a seam-sealer in teak decks. While polysulfides are specifically recommended for this application, most manufacturers issue warnings against the use of polyurethane for teak decks. 3M's 5200, for instance, carries this typical warning: "Not recommended for sealing wood deck seams because it may be permanently softened by certain teak cleaners." A notable exception to this prohibition is Sikaflex 231 which, according to a Sika Corporation brochure, is: "...formulated for use as a wood-deck-seam sealant for teak and mahog-any decks." The same brochure, however, goes on to say, "Test Sikaflex 237 for compatibility with wood cleaners, conditioners, and paints." Given the problems of having to purchase, and then test a variety of teak cleaners, this warning would encourage us to "stick" with polysulfide for bedding and sealing teak.

An occasional splash of fuel should not affect cured polyurethane, but repeated or continuous exposure will soften the material. This is another reason why polyurethane is not recommended for bedding deck seams, and why polysulfides are preferred for seals and gaskets in fuel systems.

Polyurethane is not the "ideal" bedding and seam-sealing compound. While polyurethane adhesive sealants are not "glass-like" in rigidity, a little more flexibility would be desirable for use in working seams, although their superior adhesion makes this problem more of a theoretical than a practical one. Polyurethane adhesive sealants are definitely not for bedding parts that need routine replacement, for emergency repairs, or for rush jobs. Polyurethane is, however, the material of choice for installations that are considered permanent, or where the con-struction prevents proper mechanical fastening. They can be sanded and painted, used above or below the waterline, and will adhere tenaciously to glass, wood, metal, and fiberglass. The boatowner will have to judge for himself when, and if, the super-adhesive power of polyurethane is required.

So which tube do you pick up next time on your way to the cash register? The simplest answer is: For bedding, choose between polysulfide and silicone; for seam-sealing, choose between polysulfide and polyurethane.

The polysulfides probably come closest to meeting all of the requirements of a marine adhesive sealant, and due to their superior chemical-resistance are the "safe" alternative in most cases with the exception of polycarbonate plastics. One-part polysulfide would be our first choice for the bedding of exterior teak trim, and two-part polysulfide is our choice for seam-sealing in wood hulls and decks. Slow curing is the primary drawback of the polysulfides.

The silicones are the first choice where heat-resistance is a factor, and for the bedding of mechanically fastened polycarbonate plastics. Silicone is acceptable for use underwater when the sealant is under compression, but due to its low adhesion, should not be used as a caulking compound for underwater seams. Silicone should not be used when sanding or painting will be required.

The polyurethanes are preferred where adhesion is of primary importance, and where the joint will be considered permanent. As such, we feel that polyurethanes are primarily a boat construction material rather than a maintenance material. Although some manufacturers give a qualified recommendation to their polyurethanes for seam-sealing in wooden hulls and decks, others cite greater chemical-resistance as their reason for recommending the polysulfides. In general, because items of deck hardware are routinely replaced during the life of a boat, the greatest advantage of the polyurethanes-the fact that they form a permanent bond-is also their greatest disadvantage for use as a bedding compound.

Although it is clear that no "universal” compound exists; it is equally clear that between the silicones, the polysulfides, and the polyurethanes, a long-term solution is available for all bedding and scam-sealing problems.
The above  is copied from the book "The Practical Sailor Library - Volume IV - Do-It-Yourself Improvement Projects", Belvoir Publications, Inc. Greenwich, CT, 1995. The publication "Practical Sailor" is a great resource.  It takes no advertising and reviews various sailing products and provides honest opinions!