No single wood glue is perfect for every job. Each has strengths and limitations. Choosing the right glue for your project is fundamental to a lasting assembly or a successful repair.
If all you want is a quick answer to the question, “Which wood glue to choose?” follow this guidance:
- Choose a PVA glue (the yellow stuff) for projects built with dry wood and tight fitting joints. If the project will be used outside or subjected to moisture, the glue label should include a term like water resistant, weatherproof or waterproof. “Dry” means the lumber is not just air or kiln dried but has spent at least several days indoors.
- Choose a polyurethane glue for outdoor projects with tight fitting joints when using wood straight from the lumber yard that has not been dried further. Polyurethane’s water resistance and strong bonds on end grain make it a good choice for exterior mitre joints.
- Choose a two-part slow-setting epoxy for wood projects with loose or rough joints, where gaps must be filled, and for projects that will face extreme conditions: for example, when building in cold temperatures or with damp wood, or when the project will need to stand up to a wet environment.
For a more complete answer, read on. PVA glues, polyurethane glues, and epoxies are all excellent adhesives for wood. And their capabilities overlap. A small selection from these three groups will handle a broad range of wood-gluing tasks.
When people think wood glue, they typically think yellow glue. And with good reason. This family of adhesives, the polyvinyl acetate glues, represent the most common, simple and useful general purpose woodworking glues. When used properly, they create bonds stronger than the wood itself. They are ready to go from the bottle, inexpensive, safe, and clean up with water.
PVA glues are water-based. As the water diffuses into the wood or evaporates into the air, particles left behind link to form a flexible polymer film. Manufacturers tweak the recipe to enhance attributes that glue-it-yourselfers value, including water resistance; working time, that is, the length of time the spread glue gives you to assemble, adjust and clamp your project; and initial tack, or the stickiness of the glue while liquid. Glues with higher initial tack are better at preventing parts from sliding out of position during clamping.
Keep in mind when using PVA glues:
- PVA glues require tight joinery and strong, even clamping pressure. Glue-line thickness is critical to the strength of a joint. It is a function of both how well the joint is prepared and how snugly and uniformly the joint is clamped. At the risk of oversimplifying, woodworkers aim for a finished glue line about as thick as a sheet of copy paper. In a well crafted joint, the bond line may even be thinner. But below a half sheet thickness, the joint risks being glue-starved and weak. And beyond a two-sheet thickness, the weakness of the joint grows progressively with gap size. For loose or roughly constructed joints, consider epoxy, which excels at filling gaps, or add mechanical fasteners, such as nails or screws.
Clamping, with uniform pressure along the bond line, also is important for a strong joint. Initially, the clamp force squeezes out excess glue, distributes the glue evenly and helps it penetrate. As water volume is lost, and the glue shrinks, clamp pressure ensures that the glue surfaces are kept tightly together. Minimum clamp times can vary, but typically run 30 minutes to an hour. Clamping longer is better. The joint will need 24 hours to reach full strength. Liquid glue can be wiped off with a damp rag, but many people prefer to wait for the glue to solidify and scrape it off while it is still plastic.
- PVA glues only work on porous materials. The strength of a PVA bond depends on the glue’s ability to penetratethe wood surface, where it keys in to the wood’s tiny cell structures and forms molecular bonds. It won’t hold nearly as well on most smooth surfaces.
- PVA glues are made for dry wood. For indoor projects, woodworkers typically build with lumber that has a moisture content of 6% to 8%. This also is the sweet spot for most PVA glues. Some will work well at levels as high as 10% to 12%. As a general rule, your wood projects will last longer if the moisture content of the lumber during glue-up matches the environment where the project will live. If you build an outdoor chair in a very dry workshop with very dry wood, the wood will begin to absorb moisture and swell when placed outside. This leads to problems like warping and splitting, and can place great strain on glue joints. Similar stress issues arise when projects built with damp wood shrink and dry indoors. For outdoor construction, using wood straight from the lumber yard (where moisture content often runs 13% to 20%) polyurethane glues and epoxies can be better choices.
- PVA glues aren’t structural. PVA glues remain fairly flexible, which is a plus for most wood projects. But the lack of rigidity makes PVA glues unsuited, by themselves, to structural or load-bearing applications. Under a long-term load, PVA glues tend to “creep” — that is, stretch or deform. This can have small consequences, like shifted parts that reveal their misalignment at the glue line, or large ones, like complete joint failure. Other glues are better for processes like bent lamination, where wood remains under significant tension and compression.
- PVA glues won’t take stain after curing. In projects that will have a transparent finish, and therefore visible glue lines, glue colour is an important consideration. PVA glues that dry yellow work well on pine and light woods. Others dry tan, light brown or dark brown. If the glue and wood are a poor colour match, the bond line may show. In their liquid state, PVA glues will take aniline dyes. Photo 2 shows a colour comparison of three dried glues.
- PVA glues don’t cure well in cold temperatures. It’s best to apply PVA glues at room temperature. Most of them list minimum application temperatures of 10°C to 16°C (50°F to 60°F). Titebond III can be used down to 7°C (45°F). Below the recommended minimum, glue films won’t form properly. If you need to apply glue in colder conditions, several common slow-set epoxies can be used down to 2°C (35°F).
PVA: Water Resistance and Open Time
Within the PVA family, adhesives differ in many small ways and few that are especially important to the woodworker. Primary among them is water resistance. PVA glues for indoor wood project are often identified as carpenter’s or cabinetmaker’s glues. They are very strong but not meant for regular exposure to water. Good examples include Lee Valley Cabinetmaker’s Glue 2002 GF, LePage Pro Carpenter’s Glue and Titebond Original Wood Glue.
Other PVA glues are modified to increase water resistance. These include LePage Outdoor Weatherproof Wood Glue, Titebond II Premium, and Titebond III Ultimate Wood Glue. The labels highlight terms like “water resistant” (Titebond II), “weatherproof” (LePage) and “waterproof” (Titebond III). These descriptors reflect at best small differences in real-world performance, with the edge in water resistance going to Titebond III. Any of them will work for well-sealed outdoor projects that won’t be subjected to regular soaking, and for indoor wood projects, like cutting boards, that will be wiped down intermittently with water. If you want to buy only one bottle for all your PVA needs, choose one of these modified PVA glues.
It’s worth emphasizing that water resistance is both important and easy to overstate. All outdoor glue joints will fail if subjected to repeated wet and dry cycles. Wood that swells and shrinks excessively creates forces that no glue, no matter how waterproof, can restrain in the long term. For a lasting outdoor project, seal the wood thoroughly against both water intrusion and ultraviolet radiation with multiple coats of paint or outdoor varnish; maintain this protective finish; and limit the project’s exposure to water as much as possible. It often pays to reinforce critical joints with mechanical fasteners. Know also that it often takes wood glues, both PVA and polyurethanes, up to a week at room temperature to develop their full water resistance.
Another notable difference between PVA glues is “open time.” This is the time you have to work between spreading the glue and joining the glued surfaces. If you delay assembly beyond this point, the glue will begin to dry and skin over, and fail to bond properly. Generally, the larger and more complex the assembly, the more open time matters. Most PVA glues have open times of 5 minutes or less, which is fine for small projects. Titebond III lists an open time of 8 to 10 minutes. And some PVA glues will give you 15 minutes or more: These include Lee Valley Cabinetmaker’s Glue 2002 GF, and two special Titebond formulations: Titebond Extend Wood Glue and its water resistant sibling, Titebond II Extend Wood Glue. An extra 10 minutes makes a world of difference when you are racing the clock.
Polyurethane glues can substitute for PVA adhesives in many applications. But they also have unique properties that make them superior for some jobs. They are highly water resistant, which makes them especially suited to outdoor projects. Original Gorilla Glue is the best know of the polyurethane glues, but most major brands, including Titebond and LePage, include polyurethanes in their lineups.
Like PVA glues, most polyurethanes are one-part, strong when used for close-fitting joints and require clamping. Like PVAs, they also are best applied at room temperature. At their minimum application temperatures, 5°C to 10°C (40°F to 50°F), they are harder to spread and cure more slowly. Open times may vary. Gorilla Glue has an open time of 10 minutes to 15 minutes at room temperature, and Titebond Polyurethane Glue has a total assembly time (open time plus some time after joining to fiddle with alignment) of 25 minutes to 30 minutes.
Polyurethane glues differ from PVA adhesives in some important respects:
- Polyurethane requires moisture. Unlike PVA glues, polyurethane is water activated. In wood joints, the curing glue expands into pores and reacts with moisture there to form strong bonds. This makes it a natural for gluing slightly damp outdoor woods. But it also means that dry woods, those below about 10% moisture content, need to be wiped or misted with water before gluing. Usually, water is applied to one surface and glue to the other. For dense hardwoods, both mating surfaces are dampened. A downside: Moisture inside an open bottle can cause polyurethane to harden prematurely. To extend shelf life, squeeze air from the bottle, cap tightly and store tip-down in a cool, dry place. Buy polyurethane only in quantities you can use up fairly quickly.
- Polyurethane expands while curing. A byproduct of polyurethane’s reaction with water is carbon dioxide, which causes the curing glue to foam. If woodworkers have one complaint about polyurethane, it is that the syrupy glue can be messy. If over-applied, it produces large amounts of frothy, sticky squeeze-out. The glue can expand up to 3 times its starting volume. To avoid excess foaming, apply thin layers. Uncured residue can be wiped off with a solvent like acetone, mineral spirits or lacquer thinner. But it’s usually preferable to wait for the glue to dry, remove it with a sharp chisel and sand. Because of its foaming action, polyurethane will fill gaps, but this is not structural. The foam may have a cosmetic benefit, and even seal gaps from moisture, but it adds no strength.
- Polyurethane needs tight-fitting joints. Like PVA adhesives, polyurethanes need close contact between mating surfaces and uniform clamping pressure. Clamping maintains a tight glue line and counters the force of the expanding adhesive. Unlike PVA, polyurethane dries hard. The rigidity makes polyurethane more creep proof. It also is less likely to clog or load up sandpaper. Polyurethane’s minimum clamp times are roughly comparable to PVA’s: from 30 minutes to 2 hours. And like PVA, polyurethane takes 24 hours to cure.
- Polyurethane may outperform PVA glues on end grain. Although PVA bonds are a bit stronger than polyurethane bonds in long-grain joints, there’s a good argument to be made that polyurethane has the advantage on joints that include end grain. End grain includes straw-like open cells, and more nooks and crannies than does face or edge grain. It tends to wick up water-based glues; then, as the PVA glue dries and shrinks, it may be drawn from the joint, creating a weak bond. By contrast, polyurethane is more viscous and expands, ensuring that glue remains in the joint. In practice, any advantage may be small. End-grain joints are almost always reinforced, either with nails, screws or gussets that connect perpendicular parts, or through joinery that expands the bonding surface area to include portions of long grain, like dadoes, dowels, biscuits, splines or finger joints. Nevertheless, many pros pick polyurethane for outdoor mitre joints (which are end grain to end grain) like those on deck post skirts, corners of deck top rails, and cornice trim.
- Polyurethane takes solvent-based stain when cured. This is a useful property for concealing glue joints that is not shared by PVA glues or epoxies, both of which can only be tinted in their liquid states. Before staining, or painting with latex paint, sand the glue line.
- Polyurethane will stick to most materials. Where PVA glues will only bond to porous substrates, polyurethane works on most nonporous materials as well. If you wanted to glue unfinished wooden trim to a painted birdhouse, for example, polyurethane would do the trick.
- Exposure to uncured polyurethane can cause irritation or sensitization. Dried glue is also difficult to remove from skin. For these reasons, it’s best to wear gloves when applying polyurethane glue. The glue is safe when cured.
Slow setting epoxies are versatile problem solvers. They comes in two parts, resin and hardener. When mixed, they transform into a polymer that is strong and highly water resistant. They are structural and capable of filling gaps. Most perform well in wet conditions, and some can even be used underwater, qualities that make them a favourite with boat builders.
General purpose marine epoxies are among the most adaptable. Left thin, they can seal end grain and create coatings; mixed with silica or other thickeners, they will glue wood joints, bond wood laminations and fill voids; and with the addition of dyes or pigments, they can be tinted for filling cracks and knots. By choosing different hardeners, cure times can be extended or accelerated. Examples of basic marine resins are West System’s 105 Epoxy Resin, System Three’s General Purpose Epoxy Resin and MAS Epoxies Traditional Marine Epoxy Resin. Other epoxies offer specific benefits. For example, System Three’s G-2 and West System’s G/flex are formulated to bond oily and acidic woods like teak, cedar and white oak, which are among the best woods for outdoor use. Other epoxies can be applied at temperatures near freezing.
Epoxies have their downsides, too. They are expensive. They are messy. They require accurate mixing and often the addition of fillers, which makes them less convenient than one-part glues. They pose some safety risks. And, because mixed batches have a limited life, they can create waste.
Some of the ins and outs of epoxy:
- Epoxy excels at filling gaps. While PVA glues and polyurethanes perform best with close-fitting joints, epoxy develops its full strength only with thicker glue lines. When resin and hardener mix, they generate heat, which accelerates curing and promotes the polymer cross-linking that transforms the epoxy into a tough, permanent and structural solid. If epoxy is spread too thin, as happens in very tight joints, the heat dissipates too rapidly.
Some epoxies, like System Three’s T-88, are viscous enough by themselves to fill gaps. Many require the addition of fillers, which can be used to adjust the consistency from the thickness of ketchup to that of peanut butter, depending on the end use. For general woodworking, a mayonnaise consistency works well. Typical fillers include silica powder, wood flour and glass or plastic microfibers. They thicken the epoxy, limit how much soaks into the lumber, fill uneven surfaces, and prevent sagging on vertical surfaces. For best results when using filler: Mix resin and hardener; wet the glue surfaces with the thin epoxy; mix filler into the remaining epoxy; and apply the thickened mix to the joint or void.
The gap-filling strength and long working time of structural epoxies has led some woodworkers, even custom furniture builders, to trade traditional mortise and tenon joinery for a method that uses loose dowels and epoxy. In this system, mortises are intentionally made oversized. Epoxy fills the space around the dowels or loose tenons that bridge the joint. An advantage of this system is that it allows more room for fine adjustment during assembly than would be possible with traditional tight fitting joints.
- Epoxy mix ratios are important. Each epoxy formulation requires a specific mix of resin and hardener. Sometimes it is simply 1:1— that is, equal volumes of resin and hardener. But the ratios can vary. For West System’s 105 resin, the resin-to-hardener ratio is 5:1 when using its fastest hardener and 3:1 for its slowest hardener. To simplify measuring, epoxy manufacturers offer calibrated pumps, which deliver components in their proper proportions. Still, epoxies with simple ratios — 1:1 and 2:1 — make life easier, especially when all you need is a small amount. Both System Three T-88 and West System G/Flex are 1:1 epoxies. MAS Epoxies FLAG Resin, System Three Cold Cure and System Three G-2 all use 2:1 ratios.
- Epoxy needs only minimal clamping pressure. Since the goal with epoxy is not to produce tight glue lines, clamps simply maintain parts in their proper alignment, with contact between glue surfaces, while the epoxy cures. As with other glues, apply enough that some squeezes out of the joint when clamped. Woodworkers generally use sanders, cabinet scrapers or planes to remove epoxy after it has dried. To remove uncured epoxy, scrape off the bulk, then wipe lightly with acetone, denatured alcohol or lacquer thinner. Large cured drips can be softened with a heat gun (93°C/200°F), scraped and then sanded.
- Epoxy can work on lumber too wet or dry for other glues. Most epoxies can be applied to damp wood, even where moisture content is above 20%. System Three G-2 is waterproof enough that it can be used in boats below the waterline. LePage Marine Epoxy will cure underwater. A viscous epoxy, like System Three T-88, will bond damp wood, but needs to be worked into the surface. Epoxy can also be used on woods with moisture content below 6%, which is too low for PVA glues. This is handy during northern winters, when heated air can leave wood bone dry. Just remember, joints in very wet or very dry woods have the odds stacked against them. They’ll fare best when the wood remains at a fairly stable moisture level, be that wet or dry.
- Some epoxies can be applied in cold weather. Epoxies like System Three Cold Cure and T-88 will cure in temperatures as low as 2°C (35°F) with no loss of strength. West System’s G/Flex and LePage Marine Epoxy can be applied as low as 4°C (39°F). As with most glues, the lower the temperature, the longer the cure time. For example, T-88 cures in 3 days at 25°C (77°F) but may take a full week to cure at 2°C (35°F). MAS Epoxies FLAG resin, mixed with its fastest hardener, cures in 2 days at 25°C (77°F) but will take 8 days to cure at 5°C (41°F). When possible, warm the epoxy and the surfaces to be bonded. Warming thins the epoxy, which makes it easier to mix and spread, and kick starts the reaction needed for epoxy to cure properly.
- Epoxies offer long working times but cure slowly. One of the most useful qualities of slow setting epoxies is the time they give you to assemble large and involved projects. Manufacturers offer guidance on open and cure times for each resin/hardener combination, often in relation to ambient temperature. These cure schedules sometimes appear on larger containers of hardener, but they also are available on manufacturer websites. Ambient temperature plays a large role in how long mixed epoxy is usable and how fast it cures. Epoxy cures more rapidly when it’s hot and more slowly when it’s cold. Resin systems that offer a choice of hardeners – fast, medium, slow—allow you to pick the best one for any given temperature. As an example: At 25°C (77°F) FLAG epoxy’s cure time ranges from 2 days with a fast hardener to 5 days with a slow hardener. At the same temperature, slow hardener will give you a pot life of 23 minutes, while the fast hardener will give you just 7 minutes. (Pot life is a measure of the time a given quantity of mixed epoxy provides before it becomes too viscous to work effectively. Pot life is less than working time.) Use published numbers as general guidelines. Because curing epoxy generates its own heat, working time decreases as thickness of epoxy layers increase.
- Epoxy can be coloured when wet. Epoxy’s ability to fill gaps and voids, coupled with its ability to take dyes and pigments when liquid, makes it a very popular for filling cracks and holes in wood, often in dramatic fashion. Many general purpose epoxies are amber coloured when mixed. But some, like MAS Epoxies FLAG, are clear. Often, these are mixed with sawdust, for a natural look, or tinted black for contrast. For more ambitious router inlay and casting projects, specialty casting and bar-top epoxies offer many benefits. They are formulated to be water-clear and non-yellowing and won’t significantly effect the colour of pigments. They also have other properties suited to the task. Most epoxies can be mixed with commercial tints, dyes, tempera paints, crushed stone, various powders and metallic flakes. Check with your manufacturer for specific guidance. At volumes up to 5%, decorative additives will have minimal effect on epoxy performance. A popular use is to turn natural-edge flitches or slabs of lumber into “river tables,” which feature deep streams of blue resin between banks of wood. Generally, when using epoxy for inlay, the recesses should be pre-sealed, and poured epoxy should stand slightly proud of the surface. When dry, scrape or sand it flush.
- Epoxy requires safe handling. Hardeners and uncured epoxy are irritants and sensitizers. They can cause issues like contact dermatitis and allergic reactions. Wear gloves and avoid skin exposure when mixing and applying. Work in a ventilated area, and use a respirator while sanding. Although epoxy may be ready to sand in a few hours, it can take up to two weeks to cure fully. During this curing period, the dust still contains harmful unreacted substances. Consult the manufacturer’s instructions for more-complete safety information.
Under certain conditions, epoxy can be a fire or burn hazard. Curing epoxy generates heat. This is normal and essential. But problems arise when contained epoxy is too deep, and surface area exposed to the air too small, for heat to dissipate. This can create a feedback loop: Curing epoxy retains heat, which accelerates curing, which generates more heat. If, for example, you fill a small plastic drinking cup with mixed epoxy, heat can build rapidly enough to melt the plastic and turn your epoxy into a smoky lump. According to the maker of West System epoxy, a 100-gram mass of mixed epoxy can reach 200°C (392°F). If the mass of contained epoxy is large enough, the spiralling heat can cause fires. To avoid bad outcomes, work with small batches, and apply mixed epoxy from a dish or similar container that allows the glue to spread out and dissipate heat. Not only is this safer, but it maximizes the epoxy’s working time. When filling deep voids, apply epoxy in multiple thin layers rather than one thick one.
Not too long ago, it was hard to find slow-setting epoxies in small quantities. That has changed. Epoxies like West System’s G/flex 650 epoxy and System Three’s T-88 epoxy are available in kits as small as 237mL (8 fl. oz.). West System sells handy 17mL (.56 fl. oz.) packets of its pre-measured 105 resin with hardener. And LePage offers marine epoxy in 25mL (.85 fl. oz.) resin/hardener syringes. Leftover resin and hardener rarely go to waste. They generally have a long shelf life, even after the bottles have been opened. Under certain conditions, resins can crystallize. Bathing the bottle in hot water (50°C) for a few minutes will return them to their liquid, fully usable state.
Quick Curing Epoxies
The liquid epoxies you’ll find in hardware stores are most often five-minute epoxies sold in small double-barrel syringes. They set quickly and work well for small, non-structural surface repairs. For example, when confronted with a screw hole that has become enlarged and no longer holds, you can drill it out, plug it with an epoxy coated dowel (or sawdust thickened epoxy) and drill a new pilot hole. Or you can use quick-curing epoxy to glue back a large splinter that has torn from the end of a board during sawing or routing. Five-minute epoxy also works to secure loose knots, and seal them so they won’t bleed through when painted. It’s good for places where clamps can’t reach, sticks fast to most materials and is water resistant — though usually not to the degree of slow-curing epoxies.
Other Adhesive Options
Certainly, many specialized adhesives exist that are useful in woodworking and superior for some applications. Hide glues, for example, are sometimes used in the repair of antiques and musical instruments, in part because they are reversible and lose their grip when steam heated. Contact cements are often used for bonding wood veneer to a substrate. Resorcinol glues, the go-to waterproof glue before epoxies came on the scene, remain popular with old school builders of wooden boats and aircraft and outdoor furniture. Urea formaldehyde glues are often used for wood laminations, like those used in bentwood rocking chairs and curved stair railings. Cyanoacrylate glues, or super glues, can substitute for five-minute epoxies in many small DIY repairs. They are employed by wood turners and luthiers for a host of tasks, including stabilizing cracks, filling chips and gluing inlays. For every specific need, you will find an adhesive. But the big three — PVAs, polyurethanes and epoxies — will ably handle the majority of your wood-gluing needs.