Choosing the Right Boat Glue

Any time I start researching glues, I can hear the voice of Old Man Parker from the classic film “A Christmas Story,” ringing in my head:“You used up all the glue on purpose!” And each time, the chemical engineer inside me wants to ask, “What kind of glue would fix Mr. Parker’s treasured stocking-leg lamp?” As sailors, we are all Mr. Parkers at some point in our lives—in search of the right sticky to fix what’s broke.

A boat is made of hundreds of materials, and just using “glue” to fix something is a recipe for failure, no doubt one reason why glue had a mixed reputation back in the day, and why it still fails those who have not learned the intricacies of adhesive selection and joint design. It remains a growing engineering field. We’ve used all of these and use most of them frequently. Success is a matter of picking the right tool for the job.

EPOXIES

Laminating epoxies. Examples of this type include MAS, West Systems, System 3. Formulated for laminating fiberglass, carbon fiber, Kevlar, and other reinforcing fibers, they are formulated thin to better wet-out fabrics quickly. They have high bond strength, high mechanical strength, and high stiffness so that loads are transferred efficiently to the reinforcing fibers. They can be thickened with fillers, which vary in strength, weight, and sandability. To keep it simple, we like colloidal silica for bonding and sealing core, and microballons for fairing. (Colloidal silica filled epoxy is harder to sand than gelcoat or the surrounding laminate, making it very difficult to sand and difficult to sand fair.) However, adding fillers generally weakens the epoxy, because air bubbles are entrained in the process.

5-minute Epoxies. Similar to conventional epoxies. You can add thickeners and use them for quickly tacking assemblies in place, before bonding and taping properly with laminating epoxy. But they are several times weaker and have a wicked exotherm. Use it to fill a bolt hole and it will boil right out.

Toughened or flexible epoxies. Examples include G-Flex and Dr. Sails 25. While 3-10 times more flexible than conversional epoxies, they are still 20-50 times stiffer than urethane sealants. The adhere better to plastics, including polyethylene, if properly prepared. Much of the improved adhesion to plastics comes from the ability to flex with the substrate. Excellent for many repairs requiring improved toughness, mounting equipment, and even for sail repair (see “Fixing Laminate Sails Part II,” PS May 2019). Cure times range from quick to overnight—like conventional epoxies, the slower curing products are generally much stronger.

Underwater Epoxies. Examples include JB Waterweld, Pettit Splashzone, Star Brite Epoxy Putty, and Total Boat Underwater Repair Putty. Wet your gloved hands (so it does not stick), knead it to a uniform color, and work it firmly onto a wet surface, and it will stick tenaciously and bond permanently. Amazing. We tested strength using samples applied to saturated fiberglass underwater; see “Underwater Epoxy Test,” December 2020. Thickened and designed to even work underwater, everyone should carry some for emergencies. It is also quite strong and suitable for below the waterline repairs and installing equipment in damp bilges.

Sail-Specific Adhesives. We tested Dr. Sails, a flexible quick-set epoxy (see “Sail Repair Tapes vs. Glues,” PS November 2017). We loved the short term results, but it didn’t like UV and began to fail within a year—although a year of constant UV exposure is a lot compared to real-world exposure. Plexus MA-300 and G-flex were too stiff for the job, and didn’t adhere as well.

METHYL METHACRYLATE ADHESIVES (MMA)

Toughened methacrylate. Examples include Plexus MA310 and Devcon Plastic Welder. Many newer boats rely on this product to bond keel grids, bulkheads, and hull-to-deck joints. It is also popular for bonding equipment and attaching threaded studs and gear to walls (see “Glue-on Fasteners,” PS June 2017). Though more expensive than epoxies, they are pre-thickened, require only modest surface preparation, and cure faster than epoxy. Yes, 5-minute epoxy cures quickly, but it’s not nearly as strong and is not thickened to grab and stay put.

Thread-locking compounds. This is a subcategory of methacrylates that can cure in the absence of oxygen, way down the length of the threads. They vary from intentionally weak formulas that can be broken loose with a wrench, to more permanent versions that require heat. They also prevent thread corrosion and seizing, so while it may take some effort to remove the fastener in question, it should not be seized. For example, Loctite red (271) is quite strong, but it comes off easily enough after heating with a propane torch for several minutes (OK for nuts, not so much in threaded castings). Sta-Lok rigging eyes are normally secured with 271 and some folks have used it on galvanized anchor shackles, although we wonder if these were later cut free. Nuts secured with Loctite blue (243) can be removed cold with hand tools and just a little extra force. It is common on engines, where vibration is a problem. Threads must be degreased before application for either product to reach full strength.

Superglue-type methacrylates. A specialized water-initiated cure that takes place within seconds to minutes, depending on the thickness and humidity. Gap filling ability is very poor, so always pre-fit. They don’t like porous surfaces. Normally not a permanent fix, but product-specific cements are the best choice for gluing O-rings and window gaskets that have been cut to length and formed into rings. Also for items that cannot be clamped.

SOLVENT CEMENTS

PVC Pipe Cement. Specific to PVC pipe, the solvent mix dissolves the surface of the PVC, combined with PVC resin in the glue makes a virtual weld. Also ideal for bonding PVC appearance board (see “Is Azek the New Starboard?” PS December 2021). For rigid PVC materials only.

ABS Pipe Cement. Similar to PVC cement, but different solvents and resin. Alternatively, there are all-purpose PVC and ABS solvent cements.

Polystyrene Glue (AKA model airplane cement). Specific to polystyrene, the solvents dissolve a surface layer of the substrate and fill the gaps with dissolved resin. Not many marine applications.

You would think there would be some overlap among solvent cements, but in our experience the results are poor.

ELASTOMERIC ADHESIVES

Contact Cement. Weldwood is available at most hardware stores, and 2-part formulations cure faster and are recommended for Hypalon inflatables. We’ve tested it for sail repair. It lasted about a season and was strong and easy. We’ve patched inflatables with Weldwood, Sika 291, Loctite PLS40, and proprietary 2-part glues, and we had the best durability with Loctite. But a lot of it comes down to technique and good surface preparation. Also for fabrics and leather. Very helpful for getting self-adhesive Velcro to stick to porous surfaces, including wood and fleece.

Spray Adhesives. 3M Super 77 is the best known, but Gorilla Heavy Duty Spray Adhesive seems about the same. Our only boat use is reattaching fabric (Frontrunner or similar products—not carpet) hull liners. For permanence, apply two thin coats, 5 minutes apart, press the fabric firmly into place when just tacky, and then press firmly again after 10 minutes.

Weather Stripping cement. Specialized versions of contact cement.

Polyurethanes. Because of its relatively high modulus (stiff compared to other polyurethane caulks) and good bond to fiberglass and gelcoat, 3M 5200 is generally classified as an adhesive, not a sealant. On the other hand, many more flexible sealants adhere just as strongly to some surfaces, and more flex may be what you are looking for. See “Marine Sealant Adhesion Tests,” November 2016.

Silicone. Primarily a sealant, but superior UV resistant makes it the go-to adhesive for securing glazing. DOW 795 is the ONLY silicone we recommend for this purpose; we’ve used other silicones and the glazing popped out in time. Other formulations offer superior oil and grease resistance, making them first choice for gasketing lubricated mechanical equipment. We recommend Permatex RTV silicon; black for most fuel and lube related uses and red for high temperatures. Avoid hardware store silicones unless you are ready to test them first.

Shoe Goo. Originally developed for repairing holes in the toes of tennis shoes, this is styrene butadiene rubber (SBR) in solvent. I’ve used it on shoes, even sailing; I tend to wear out the little toes area lounging at the helm. For gluing soles back on, Loctite PL-S30 or Sika 291 are better choices. Shoe Goo is great for chafe on the corners of high-mileage luggage (smear it into the fabric). I’m sure it has some boat uses, and we’re told it’s good for sealing wires. It’s tenacious, but UV resistance is limited.

WOOD GLUES

PVA (polyvinyl acetate). This category includes everything from Elmer’s White Glue to Titebond II, our favorite water resistant glue for cabin cabinetry. It’s easy to work with, strong, and water resistant enough for anything that is not likely to be submerged.

Resorcinol. Before epoxies, resorcinol glues where the go-to adhesive for laminating wooden spars and most boat joinery. Many still swear by it for marine woodwork, but its gap filling ability is very limited, so you need to be a good carpenter and make tight joints. Aerodux 185/155 from Spectrum Adhesives is well respected.

Foaming polyurethane. The original Gorilla glue is chemically akin to the elastometric polyurethane sealants, but it cures to a more rigid state, like a polyurethane foam. There has been a lot written about how strong the original Gorilla Glue is, its ability to fill gaps, and also, about how weak it is, particularly outdoors.

Gorilla Glue can be strong if the joint is tightly fitted and clamped tight while curing. However, it foams in the presence of moisture, and it can force the surfaces apart, and any gaps that result will be filled with a weak foam. This also makes it susceptible to freeze/thaw damage if it gets wet.

We’ve taken advantage of Gorilla Glue’s foaming trait to repair certain minor types of core damage, since polyurethane is moisture tolerant and stronger than other foams (see “Can Glue Injection Fix Rotten Core,” PS July 2018). This technique should not be used as a substitute for a properly fitted joint. If the joint is tight, we prefer Titebond II in most cabinetry applications, or epoxy where more gap-filling capability is required.

I still want to know what kind of glue was “used up on purpose” in “A Christmas Story.” If only we had been there, equipped with the magic bonding agent for the lamp’s leg and the necessary reinforcing materials, perhaps the Old Man would have settled down. We can’t go back and fix what’s in the past, but with the right glue we should be able to fix whatever we’re going break today.

VALUE GUIDE: COMPARING ADHESIVE SEALANTS