Adhesives play an important part in woodworking. Of all the joinery techniques in existence, adhesives are one of the most common. While nails and screw are in common use, especially for building buildings, adhesives provide superior bonding, as well as the advantage of not leaving any hole behind or hardware visible.
There have been a variety of different glues used in woodworking throughout the years, almost all of which are still available today. But PVA glues have become the number one choice for woodworking, due largely to their availability, price and ease of use.
So Just what are PVA Glues?
The term “PVA” stands for polyvinyl acetate, a member of the polyvinyl esters family, and actually refers to a variety of different glues which are all similar. This includes wood glue, white glue, school glue and carpenter’s glue. The differences between these various glues are minor, but can affect how they work.
Looking at it another way, PVA glues are a thermoplastic suspended in water. When the water evaporates away, it leaves the plastic, the molecules thereof which are joined together, as well as having entered into the pores of the wood or other material, creating a strong bond.
Different PVA adhesives will have slightly different properties. Probably the most notable of these is the differences between white and yellow PVA glues. White PVA, such as school glue, is not waterproof at all, can be weakened over time and can yellow. Yellow PVA is water resistant, although not waterproof (it can’t be immersed), so moist does not affect it. However, yellow PVA is more expensive than white, which is why white glue is used for craft projects and in the classroom.
Part of the motivation for creating PVA glues came from the difficulty in working with hide glue, which was the common glue in use for woodworking in the years before PVA glues existed. Hide glue needs to be kept heated in preparation for use and while being used. It also produces an odor which some people find objectionable. PVA on the other hand can be used at room temperature and is odorless. On the other hand, hide glue can be removed by heating, which doesn’t work for PVA.
Unlike epoxy, which is set by a chemical reaction, PVA glues are set by evaporation. Polyurethane adhesives, like Gorilla Glue and Cyanoacrylate glues, like Super Glue also cure by a chemical reaction; but in the case of these adhesives, it is moist that triggers the chemical reaction, not something contained in the adhesive itself. This means that shrinkage can be more of an issue with PVA glues, than it is with any of these others. Where high gap filling is necessary, PVA glues are not the best choice, as they are not a high solids adhesive.
However, PVA is water soluble, making it possible to clean it up with water. Adding water to the glue and thinning it out is useful in some situations and with some materials, although not usually with wood. Adding water can also be used to salvage old glue that has thickened in the bottle. In either case, add small amounts of water at a time, stirring each time more water is added, until the desired consistency is reached.
Please note that because PVA glues are water-soluble, they can be cleaned out of clothing or carpeting, as long as they are still wet. Once dry, cleaning them out requires soaking the fabric for an extended period of time to soften the glue and then scrubbing the glue with a stiff nylon brush to break it up.
Once dry, PVA is both strong and flexible. Typical tensile strength for PVA glue is 3,750 PSI, which is roughly half that of epoxy. However, in any properly glued joint, the wood itself is more likely to break than either of these adhesives.
Disadvantages of PVA Glue
While PVA adhesives are excellent, they are not without their failings. For most of us and most of our projects, these failings are minor enough so as to not be important. Even so, we need to be aware of them, so that we don’t expect something of the glue that it can’t deliver.
To start with, PVA is water based, that means that it should not be allowed to freeze. Freezing breaks up the polymer (a string of the same kind of molecules) destroying the glue’s ability to stick. Allowing the water to evaporate out of the glue, either through it sitting too long or the lid being left off the container also renders the glue useless.
While PVA adhesives do dry fully, it takes 24 hours. This is confusing to some people, because the working time is short, a mere 3 to 5 minutes. But full curing requires all the water to evaporate out. While this might happen quickly on the surface, cutting short the working time, it takes considerably longer for the moist inside the glue to work its way out into the air.
There are some algae, bacteria, fungi, lichens, and yeasts that can break down PVA adhesives, degrading their holding power; however, I have never had this happen with any of my projects.
PVA Glue and Finishes
Care must be taken when using PVA glue, so as to keep it from getting on finished surfaces. Some people say that PVA should not be used in cases where the project is going to be varnished. This comes from the problem PVA causes for stains and varnishes, in that it both fills the pores on the surface of the wood and skins over the top of it. When stains or varnish are applied over this, it makes for a splotchy finish.
Some people say that the solution to this problem is to wipe the glue off with a damp cloth or paper towel. This will get rid of any glue that is sitting on the surface, but will not do anything to get rid of the glue that has made its way into the wood’s pores. Others say that the best thing to do is to allow the glue to bead up and then remove it with a chisel once it is dry. This works better, especially in cases where there is only a small amount of glue. However, it is not a foolproof solution, as the area under the beaded up glue will still be a problem. That glue will need to be sanded off, to access bare wood for finishing.
Of course, this isn’t an issue with opaque finishes, such as paint. When in doubt, do a test piece, before assembling the project, so as to see how the glue might affect the finish.
Preventing Glue Squeeze Out
Obviously, the best solution is to not get the glue on the finished surfaces. That can be challenging, especially since it is usually necessary to get the glue onto the entire surface being joined together, that generally leads to some of the glue squeezing out of the joint.
This can be prevented through, during the gluing process. One key is to learn just the right amount of glue to use for various types of joints, applying what’s needed and no more. Holding back just slightly from the edge, such as in a miter joint helps as well, as the glue that would squeeze out has a place to go. Finally, in more difficult cases, such as when putting a mortise and tenon joint together, bring the joint most of the way together and then stop to wipe the excess glue out of the joint with a toothpick, before closing the joint all the way.
Planning for Squeeze Out
Another option is to plan for the potential of glue squeezing out by partially finishing the pieces, before assembly. If a project is to be stained and varnished, it can be easier to apply the stain and the first coat of varnish before assembly. Then, when the project is assembled, any glue squeeze out is going to be on top of that first layer of varnish, where it can be cleaned off, either with a damp cloth or a chisel, leaving a surface that can accept additional layers of varnish, without staining.
Using PVA Over Existing Finish
This raises another question – can PVA adhesives be used over finishes, such as varnish or even paint? The answer to this is yes… and no. It’s a yes because the adhesive will bond to those surfaces, allowing things to be glued together. But at the same time, the no comes because the resulting glue joint will not be anywhere near as strong as what would normally be expected. Chances are that if the joint was put under pressure, it would come apart.
Part of the way that most adhesives work requires that they get a good “bite” on the material surface. In the case of wood, this is provided by the pores in the wood. We can’t see it, but a small amount of the glue oozes into those pores, so when the glue dries, there are continuous strings of polymer from inside the pores out to the rest of the joint. That’s an important part of what provides the strength. When those pores are already blocked by finishes, the glue joint is not as strong.
One thing that can be done in these cases is to rough up the finished surface with medium to coarse grit sandpaper, providing scratch lines for the glue to “bite” into. The resulting scratches should be limited only to the area that will be glued, so as to not mess up the piece’s finish.
Using PVA Glue
As with any adhesive, proper surface preparation is essential for using PVA glues. That means having a clean, flat, oil free surface. PVA has very minimal gap filling capability, so it is important that the surfaces to be glued together fit together cleanly and snugly. If there are any gaps, the parts will either need to be sanded to eliminate them or another type of adhesive should be used.
If the surfaces fit together well, but are extremely smooth and non-porous, it can help to sand them slightly with coarse sandpaper, in order to ensure that the glue has something to bite onto. Then remove all sawdust from the surfaces to ensure that the sawdust doesn’t get in the way of the parts coming fully together.
Always take the time to dry clamp the project together, before gluing it. PVA glues only have a 3 to 5 minute working time, so it is important to be able to clamp the project together quickly. Dry clamping provides the opportunity to ensure that everything fits, there are enough clamps available and that there aren’t going to be any unforeseen problems clamping the parts together. Cauls may be needed in some cases, such as when laminating a tabletop or gluing curved pieces together.
Apply the glue to one of the surfaces to be joined together. Glue bottles come with a nozzle to make this easier and it is, in most cases. But there may be times when large areas need to be glued together, where the nozzle is not an advantage. In those cases, it could be better to either pour the glue onto the surface or work out of a cup. The glue can be spread:
- With a finger, the best glue spreader there is
- With a plastic glue spreader
- With a paintbrush (great for large areas)
- With a paper towel or rag (although this wastes a lot of glue)
Press the surfaces to be glued together and clamp them. Enough clamping force needs to be used to force the parts together, closing the gap between them. Allowing just a bit of squeeze out can prove that the parts are as tightly clamped as possible. Don’t over-tighten, as that can damage the wood or cause warping of the project.
Allow the project to sit for 24 hours to dry, before removing the clamp.
Chip off any glue bead that squeezed out of the joint with a sharp bench chisel and sand the joint smooth.
Finish the project.