In matters of science and cutting-edge experiments, especially dangerous cutting-edge stuff, ethicists tell us we should ask two questions: can we do this, and should we do this? Dangers can lurk in experiments, and we don’t want to test a theory that could result in those dangers becoming reality. It’s the stuff that science fiction movies and books present often.
Well, we’re woodworkers, not scientists, and the stuff we work with is not dangerous except to ourselves if we are not careful. In this instance, and asking the question can we screw into wood glue (see that little rhyme?), the obvious answer is sure, we can. But the more important question is should we screw into wood glue.
There are circumstances where we should, and there are circumstances when we can but don’t really need to since the glue will be a sufficient bond.
The Use of Wood Glue
We all have wood glue in our woodworking shop and use it regularly in our woodworking projects. It creates a strong bond that is actually stronger than the wood itself. There are three basic types of wood glue:
- Polyvinyl acetate (PVA), also known as wood glue, is one of them. The most well-known PVA glue is Elmer’s Glue. We all used it in school, and now we use it in our woodworking shops.
- Polyurethane, which we usually call Gorilla Glue, is another. It is so often referred to as Gorilla Glue because that was the first company to introduce polyurethane glue to the American market. This came along after I was in school, so Elmer’s is the one I remember.
- Aliphatic glue is the third. It is a synthetic glue, and the most well-known of the aliphatic glues is Titebond.
Gorilla Glue’s product line has introduced a PVA glue, Gorilla Wood Glue. It’s a water-based glue, reliable, and very strong. It has a thick consistency that makes it easy to apply and spread without running. It works well with all kinds of woods and wood products, including hardwoods, softwoods, and composites. It does not expand; it dries quickly and will leave a clean line.
It is not waterproof, but it is highly water-resistant. If you need waterproof, choose Titebond III instead. But, if water-resistant is all you need, you’re safe with Gorilla Wood Glue.
How Strong Is Wood Glue?
Wood glues are developed to penetrate the grain and wood fibers. It really does hold tightly and well and, in most cases, will be stronger than the wood itself. Most wood glues can withstand a shear strength of about 3500 pounds per square inch.
Stronger than the wood itself sounds impressive, but wood is not especially strong in and of itself, and sometimes other fasteners, mechanical fasteners, like nails and screws, are also used. Glue, like Gorilla Wood Glue, has its own inherent strength. When applied and allowed to dry, the wood glue is absorbed by the surrounding wood fibers, lending its inherent strength to the wood.
The math is simple: 1 + 1 is 2. The combination of glue and wood fibers is stronger than just the wood fibers.
In this regard, glue is actually stronger than screws and nails – glue is used along the entire span of a piece of wood, whether face to face or edge to edge, when gluing pieces of wood together, while a nail or screw will be strong only in the exact spot where it is placed. We wrote an earlier piece on the subject of why wood glue is so strong.
Wood glue is stronger than screws; screws are stronger than nails, so wood glue is much stronger than just nails. As regards Gorilla Wood Glue, we also wrote a piece specifically about its strength.
In the case of creating a tabletop, and as you know, we glue pieces of wood together edge to edge and tightly clamp them to allow time for the glue to dry. While the tabletop may bear weight when being used, there is usually a frame beneath that supports it. No mechanical fasteners are needed. Excess glue is wiped away, the tabletop is sanded and finished, and a strong bond is created with just the glue.
Gorilla Wood Glue is a good choice for these tasks. Easy to control its application due to its thick consistency, it’s great to work with and creates a very strong bond. It’s also water-based, meaning if thickness is a hindrance to your task at hand, a little water will thin it out for you, and as long as it’s just a little, the pieces of wood being joined will hold together well when it dries.
When Would We Use Screws With Wood Glue?
Sometimes a screw will become loose – the wood fibers around the threads of the screw have become torn away and the screw stops doing what it is supposed to do. It might be from wear and tear, or it might have been a sudden shock to the joining, but the screw stops holding pieces of wood or something attached to the wood together.
An example of wear and tear might be screws that hold a door hinge plate to the door frame. Doors are opened and closed perhaps thousands of times, and that kind of wear and tear can eventually cause the screw to become loose. The door will stop closing smoothly or tightly as a result, and something needs to be done.
In these instances, a larger screw, one that has a coarser thread (fewer threads per inch) and is larger than the one being replaced, is not an option. the pre-drilled hole in the hinge plate will limit the size of the screw you can use. So, we need to find an alternative for this kind of repair.
Wood glue will come into play, along with other materials.
Matchsticks, Toothpicks, Glue, And A Screw
Matchsticks and toothpicks are both made of wood. Take the head off the matchsticks, and they are basically the same.
You can use a little Gorilla Wood Glue, or the glue of your preference, in the screw hole and insert a few of either. Allow the wood glue sufficient time to dry, and you will have new wood fiber strongly bonded to the wood of the door frame, cabinet frame, etc. Instead of using a larger screw size, you have made the screw hole smaller. Cut off the tips of the matchsticks or toothpicks and sand the wood surface smooth.
Line up the hinge place, drive the screws in, and the hinge plate will hold tightly once again to the door frame. Be careful not to drive the screw in too hard or deep, or you will run the risk of damaging the wood fibers once again – just tight enough to hold well.
Fill The Screw Hole In Other Ways
A hardwood dowel could also be used, and glue would be used in this instance, too. The same principle applies, and the same steps should be followed. Allow the glue to dry fully, cut off the top of the dowel, and sand it down smooth with the wood’s surface. Drive the screws, and you’re done.
If you have scraps of wood handy, you can also fill the screw hole with bits and pieces of the wood mixed with some Gorilla Wood Glue. Don’t be stingy with either the wood or the wood glue – you can always trim down the wood and sand away excess glue after it has dried. With new wood fibers for the screw to grab hold of, simply drive the screw in.
Particle Board and MDF
We also work with particle board and MDF (medium-density fiberboard), and we know there can be tear-out when screwed joints are stressed beyond their ability to withstand. The matchstick/toothpick solution is not the best one for this problem.
You will want to patch the particle board of MDF first by drilling out a hole larger than the one you are repairing, maybe a 1/2″ hole, and fill it with glue and a 1/2″ hardwood plug or dowel. Allow plenty of time for the glue to dry – Gorilla Wood Glue containers will have those instructions on them for a full curing, maybe upwards of 24 hours to be sure it is solidly set.
Then, drill a pilot hole in the plug or dowel and drive the screw in. With a little patience, a little attention to the board, and the problem is solved.
The Strength of Wood Glue
We’ve mentioned the strength of wood glue and how a glued wood joint will be stronger than the wood itself. Here’s a video that will demonstrate this truth.
The wood broke a few times as pressure was applied, but the glue line held. Unscientific in the video, yes, but you’ll see for yourself how one of us woodworkers conducted his own test very effectively.
All is not lost when screws come loose and stop doing their job. Involve a bit of glue, whether Gorilla Wood Glue (which we like) or some other brand, with your screw(s), and the problem will be easily solved.