I regularly use only four or five kinds of glue in my workshop. Each glue has a specific purpose. Knowing which glue to use, and how best to use it, comes with understanding and experience. In this first of a three part series we'll look at some of the more useful glues available to woodworkers. In future articles I'll cover some useful techniques for applying glues.
PVA Glue - The All Purpose Glue
Polyvinyl acetate (PVA) is, by far, the glue most frequently used in woodworking shops. These are the common white and yellow wood glues, and should form the staple of your glue inventory. There are three general types of PVA glues: white, yellow and cross link (Shown above: LePage PVA glues)
White and Yellow
White and yellow PVA glues are simple to use. No mixing required – just apply, clamp, and allow to set. Open assembly time (how much time you have before the glue sets up) is short - about 3 to 5 minutes for white glue, and 5 to 6 minutes for yellow. (Note: these times are somewhat lower than specified by manufacturers, but they reflect my experience. The environment in which I typically glue up is 20C and 50% relative humidity). Yellow glue has a higher tack level than white glue, making it easier to keep parts aligned before applying clamps ('tack' refers to the initial grab or stickiness of the glue before pressure is applied). Yellow glue is also easier to sand than white glue.
PVAs have little or no gap-filling ability, so joinery must be good. They bond within the structure of the wood, not on top, so it’s impossible to "starve" a joint unless you don’t apply enough glue to penetrate into the cell structure on both sides. You can clamp tightly down to a zero-thickness glue line – in fact, I recommend it. Some yellow glues dry transparent and others dry amber, while white glue dries clear. Both clean up with water. If you don't use a lot of glue, buy it in small (half litre) quantities, particularly for yellow glue, which has a 1 year shelf life. White glue has a longer shelf life of 2 to 3 years.
I prefer yellow PVA and use the Lepage brand for over 90% of my gluing. It is an easy-to-use water-based glue that is inexpensive and works well. Clamp for a minimum of 30 minutes (I prefer two hours or so), and don’t stress the joint for 24 hours.
There are variations of PVA glues that offer water resistance, slight gap-filling ability and slower set times.
Regular white and yellow PVA is not designed for outdoor use and won’t resist moisture. For projects that will be subjected to high humidity, or even occasional surface wetting such as patio tables, one part cross-linking PVA is available, such as Titebond II. In general these are referred to as 'Type II' glues. These are a little more expensive than ordinary PVAs. Cross-link glue has a 5 to 10 minute open time and requires about 1 hour clamp time. They dry dark, like yellow PVA, and have a 1 year shelf life. Clean up with water.
Cross-linking PVA is not for complete water submersion – for that, see epoxy glue below.
High Solids PVA
High solids PVAs include Lee Valley’s 2002 GF Cabinetmaker's Glue, which I regularly use, and Titebond III. These apply like regular PVA but offer longer open time and require just slightly more clamping time. Open time will vary from about 10 to 20 minutes, which is a great help for difficult glue-ups like large table tops, or carcass assembly. Clamp time for the Lee Valley glue is around 60 minutes, and 30 minutes for Titebond III. Both clean up with water, and dry light brown.
The high solids content (45% for Lee Valley and 52% for Titebond III) also provides some gap-filling ability. Still, this is no reason to let your joinery get sloppy and count on the glue to save you. Slight gap-filling ability is nice, but learn to make your joinery tighter. Where true gap-filling ability is required, use epoxy glue.
Titebond III high solids PVA glue
Epoxy is expensive compared to any of the PVA glues on the market. You must mix two parts because they cure by chemical reaction, not by evaporation, and fumes are released as they cure. Epoxy sticks to almost anything, making it somewhat more difficult to work with.
So why use it? Firstly, epoxy is not just water resistant (like cross-linking PVAs), but it is water proof. Epoxy, then, is the best choice for items submerged in water or left in a wet environment regularly. If I were to build a garden bench exposed directly to rain, I would choose epoxy over cross-linking PVA. Cross-linking PVA might very well survive, but epoxy gives extra insurance against failure.
Secondly, epoxy is able to fill gaps perfectly. When building a cedar strip canoe you’ll never align the strips with joint-like precision as you should when edge-gluing a table top. So epoxy not only water-proofs the joints in a canoe, but also fills those tiny gaps.
Epoxy requires some glue-line thickness for its strength, as it bonds on top of the wood, not within the cell structure. Manufacturers recommend a glue-line thickness of .003", which is startlingly large if you see what that actually looks like. Tight clamping, resulting in squeezing almost all epoxy out of the joint, gives a weak bond. This is another benefit of epoxy – no clamping is technically required at all, except to bring the two parts into contact and hold them there.
Epoxy also has excellent creep resistance, which means it will withstand lateral pressure without allowing the joint to stretch. White and yellow PVA glue won’t hold up in a bent lamination. Also, getting zero-thickness glue-lines in a bent lamination is unlikely. Epoxy is my glue of choice for bent laminations for both of these reasons.
Another reason to use epoxy: it can bond dissimilar materials, such as metal to wood, or plastic to metal. Epoxy will not bond to wax or certain waxy plastics, so if you do want to join dissimilar materials, check the manufacturer’s instructions.
Regular epoxy requires pre-cure time. That means mixing the two parts well for 5 minutes and allowing the mixture to chemically react for about 15 minutes before use. Open time is about an hour and joints should be undisturbed for 24 to 48 hours. Full strength isn’t reached for about 7 days.
I also use 5-minute epoxy for really tiny glue-ups where I don’t want to wait very long. Mix the two parts, stir for a minute (at most), and apply immediately. It is just slightly gummy after 5 minutes, but quite hard after about an hour. Due to its high solids content and great gap-filling ability, 5-minute epoxy is my glue of choice to fill natural holes that you might find in a knot, or worm hole. You can add fine sawdust to the epoxy to match the colour of your wood. Epoxies generally have a 1 year shelf life, clean up with acetone, and dry clear.
There are other special epoxy types for oily or acidic woods (e.g. rosewood and teak), which typically don’t glue well with PVA. There are even "cold cure" epoxies for application in cold temperatures and even under water!
Industrial Formulators and LePage Epoxies
Hide glue is commonly used to restore antiques. One of its best properties is that new hide glue sticks to old hide glue, unlike PVAs. That's why loose joints on antiques originally assembled with hide glue can be re-glued without paring the joint back to bare wood.
Another nice thing about hide glue is that parts glued together with hide glue can be separated with steam or hot water. That is why antique and instrument restorers like this glue. Antiques can be disassembled for restoration then easily reglued.
Typically, to prepare hide glue, granules are melted in a glue pot with water. However, you can buy cold, pre-mixed hide glue. It has a shorter shelf life but still works well. Hide glues typically have a 10 minute open time, clean up with water, and dry translucent. Titebond Liquid Hide Glue is a good choice.
Titebond liquid hide glue
Hot Melt Glue
Other than for small crafts, hot melt glue isn’t used very much in woodworking. One notable exception is in applying commercial edge-banding to plywood edges. The edge banding comes in rolls with hot melt glue already applied on the back. A hot household iron is all you need to heat the edge-banding and melt the glue.
Hot melt glue comes in sticks for an electric glue gun. It has little strength so requires a thick glue line to hold.
Some woodworkers use it to temporarily attach templates to a work piece and then either physically break the bond or re-melt the glue-line with a paint stripping gun. Hot melt glue is also used to attach small moulding and trim pieces.
There is a new type of polyurethane hot melt glue on the market, the HiPURformer Advanced Bonding System from Titebond. This waterproof adhesive comes in four formats: 30, 60, and 75-second, and 5 minute set time glues. They have bonding strengths of from 900 to 1360 psi, compared to 2,800 psi for white PVA (after 24 hours curing). They dry opaque.
Bostik 23G hot melt glue gun
Titebond HiPURformer glue system
Other Glue Types
There are a few other glues used in woodworking. One is cyanoacrylate (CA) glue. Cyanoacrylate glues are known commercially as: "Crazy Glue"; "Hot Stuff"; "Super-T" and so on. These are quick setting glues used to repair small hairline cracks. Woodturners like to use them where small checks appear while turning. Such glues are also popular with model builders for attaching small parts. Clean up is with acetone.
Another common glue is polyurethane glue (PU), such as "Gorilla Glue". It is water proof and will work on many materials. It requires moisture to cure, so wiping the surface with a damp cloth before application is suggested. PU glues have a 20 minute open time, and need to be clamped for about 4-5 hours. They expand when curing, but have low gap filling properties. Clean up is with mineral spirits, and they dry yellow. Once opened the glue has a maximum 12 month shelf life.
Contact cement is yet another commonly used glue. It has very little creep resistance and not a lot of strength.
However, it does have a long open time, so it is not generally used to join wood. One of its more common uses is in applying plastic laminate to particleboard countertops. Clean up is with mineral spirits.
Loctite cyanoacrylate glue
System Three and Gorilla polyurethane glue
LePage contact cement
Although there are quite a number of choices out there regular PVA wood glue will serve the majority of your needs. Use cross-linking PVA for its water resistant qualities and high solids PVA for its longer open time. For bent laminations or projects submerged in water or subject to long periods of high moisture, use epoxy. As well, use epoxy for its excellent gap filling ability where a perfect mating of the two parts is near impossible. Finally, use hide glue for antique restoration and hot melt glue on the back of commercial edge-banding.
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