Quality joinery is the defining mark of the experienced woodworker. We start learning joinery with the very first project we make, putting two pieces of wood together and spend the rest of our woodworking career improving upon that. There are so many different types of joints to make, many of which require some real practice to develop the skill to do them well, that we can spend years trying to become even marginally proficient at them all.
Joinery also means learning how to work with wood glue, although that shouldn’t take us as long. But what do we mean when we use the term “wood glue?” There are actually a number of different wood glues that we might use and while the average woodworker has a favorite that they reach for on a regular basis, there are always projects which require other types of glues.
By and large, the most common wood glue that most of us use is PVA glue, which stands for polyvinyl acetate. Many common glues fall into this category, including white school glue. The original Titebond and Titebond II wood glues are PVA glues. But the main difference between these yellow PVA glues and white glue, such as Elmer’s glue, is that they have lower water content. That’s important to woodworkers, as high water content can cause swelling and warping of wood, as well as affecting the glue’s gap-filling capability. While yellow wood glue really isn’t considered to be gap filling in the same way that epoxy is, it is better at it than white wood glue.
Types of Wood Glues
As I already mentioned, there are several different wood glues, in addition to PVA glue. The most common ones are:
- Titebond III – The main difference between this and the other Titebond wood glues is that it is waterproof. Titebond II is water repellant, but not waterproof.
- Gorilla Glue – This is a polyurethane glue, which is intended to be a general purpose glue, rather than just a wood glue. It adheres to a wider range of substrates that Titebond III will. As with all polyurethane glues, it is moisture activated, which makes it ideal for use with wood that has high moisture content. However, it will foam due to the moisture, making it come out of the joint. That provides excellent gap filling capability, but it can be difficult to clean up.
- Titebond Extend – A slower setting version of the original Titebond wood glue. This is used in applications where complex clamp-ups are needed, as it offers a longer assembly time.
- Epoxy – Although not normally considered wood glues, epoxies are excellent for use in woodworking, especially in the case of gappy joinery. The high solids content of epoxy makes it an excellent choice where gaps need to be filled, without losing strength. All epoxies are high tensile strength. They are also useful as a finish or where potting wood might be required, such as in the making of a “river” tabletop or for woodturning. Epoxy can also be used as a high build clear finish.
- Cyanoacrylate Adhesives – Commonly referred to as “superglue,” although that is a specific brand of CA glue. This category of adhesives is excellent for repairs of all types. They are extremely fast-setting and the gel versions provide excellent gap filling. Cyanoacrylate adhesives are also used as a hard finish, especially for string instruments.
- Dry Hide Glue – This type of glue harkens back to the old days of woodworking, where cabinetmakers kept a glue pot to keep their hide glue hot and pliable. It is literally made from animal hide and is the only wood glue that is designed so that it can be heated and the parts disassembled. Although not in common use today, it is still available and is excellent for veneering work.
- Contact Cement – A high-tack glue used predominantly for the application of veneer. What makes contact cement so good for veneering is that once it tacks and the glue makes contact with itself (on another surface) it bonds instantly. Extreme care must be used when working with contact cement, but there is no risk of the glued veneer coming up once pressure is removed.
- Construction Adhesive – Normally only used in construction work, construction adhesive is a heavy-bodied adhesive, sold in caulking tubes. It is an excellent gap-filling adhesive, which is why it is used. However, it doesn’t bond with the wood surfaces the same way that other wood glues do, soaking into the pores of the wood. So while it might hold parts together almost instantly, it does not form as strong a bond as wood glues do.
Choosing the right glue for the application is an important part of woodworking. Common yellow PVA “wood glue” will work for most projects. But knowing the other types of wood glues and when to use them is an important part of any woodworker’s mental toolbox.
Clamping Wood Joints
Proper clamping is an important part of any glue-up, perhaps even more important than the type of glue selected. Clamping does three important things for the woodworker: it puts the wood pieces in proper position in relation to each other; it eliminates gaps between the pieces, so that the glue doesn’t have to span those gaps; and it ensures that the pieces remain in alignment with each other while the glue dries or sets.
Proper clamp requires that sufficient pressure is applied to ensure that gaps in the joint are fully closed. Different types of clamps provide different levels of clamping force, so it is important to ensure that the right clamps for the project are selected.
Clamp can be simple or complex, depending on the project being made. By and large, it is better to do several simple clamp-ups, than it is to perform one complex one. Even so, there are times when a complex clamp-up is required, due to the complexity of the project.
In order to do multiple clamp-ups, rather than a more complex one, it is necessary to have a very good understanding of how the project fits together. In many cases, there is a particular order in which parts need to be clamped together into assemblies. If that order is broken, we can find ourselves in the position where we need to glue parts that can no longer be put together, because of other joints which have been glued.
Understanding the working time of any specific wood glue is important to using it. When the working time is exceeded, it weakens the joint created with that glue. Dry clamp of a project is an excellent means of determining the clamping order, verifying that the joints fit together properly and ensuring that sufficient clamps are available to complete the clamp-up. Sometimes, complex clamp-ups have to be broken down into several simpler stages, simply because there aren’t enough clamps available to complete the more complex clamp-up.
Glue Dry Time
Once wood is clamped, it should be left untouched until the glue is fully set. This doesn’t just mean surface drying of the glue, but drying all the way through. As most of the glues mentioned above air dry, full drying of glue can take several hours. By and large, the only exceptions to that are the cyanoacrylate adhesives family and the epoxy family. Epoxy is unique amongst all glues that are used in woodworking, in that it is cured by a chemical reaction, rather than drying by evaporation.
For air-drying wood glues, such as the more common PVA adhesives, a basic dry time of 30 minutes is normal. But that is assuming that the parts will not be under load or stress. For full drying, so that the joints can be put under load or stress, it is necessary to give the joint 24 hours. That means that the project should be left clamped for the full 24 hours.
Allowing a full 24 hours of clamp time before any secondary operations is critical to avoiding problems with the project coming apart during those operations. We can include cutting, drilling, planning and sanding in this list of secondary operations. While there are some adhesives which can fully dry quicker than that, this is true of almost any type of wood glue, regardless of the type. If the project is unclamped or the work is attempted during the dry time, there is a high probability of the project coming apart, even if only partially so. This creates a much more serious problem than just regaling the pieces back together, as the resulting joint will not be as strong as the original one would have been.
Please note that sanding glue which isn’t dry grinds particles of the glue into open pores in the wood. This can have the additional effect of closing off those pores, affecting the appearance of finishing operations. Both staining and varnishing require open grain in the wood and anything that closes off the grain is likely to cause discoloration in the area affected.