Based on the comments that I receive on many of my videos, glue strength when joining wooden parts is widely misunderstood . This is especially true for those just starting out in woodworking, but not limited to that group. Misconceptions about glue in general, and problems with earlier glue formulations still get mentioned today, even though they are a thing of the past. For the most part, the glue that is available today can be counted on to perform as it should.

First off, I’ll point out that I am not an expert on glue. All I can do is speak from my experience using various types over the years I’ve been doing this. And this is not my first time writing about this, I went into some detail on glue in this blog post from a few years ago. I won’t echo the things I said in that post, so I urge you to read that before going forward.

In this entry, I’m going to talk about just three types of glue that are used (or, more correctly, I use) for joining wood to wood, and I’ll start with “regular wood glue”.

Yellow Wood Glue

The yellow variety of PVA glue is most commonly used for woodworking, at least here in North America. This is a very strong glue when used correctly. In fact, it can create a bond that is much stronger than the wood itself. And that’s any kind of wood – soft or hard. It is most effective for face-grain to face-grain joints, but also works well for cross-grain joints that are not overly large. It is not very effective for butt joints – end-grain to end-grain or face-grain. Joints must be clamped tightly to achieve maximum strength.

It is a liquid that contains water and while not especially thin, it is absorbed into the fiber of the wood. This property increases the bond strength of face-grain to face-grain joints, but can do the opposite on end grain joints, since end grain will more easily absorb and possibly pull too much of the glue (or, more correctly, moisture) out of the joint. If you can get enough glue into the butt joint to keep that from happening, you will have a joint that is basically only as strong as the glue is itself.

And that’s where the other two types of glue come in.
I’m not going to quote figures as for the actual number in pounds per square inch (or megapascals) for the bond strength of these glues, since that’s beyond the scope of this article. Numbers are of no importance if you can’t put them into the context of what you are doing. The three types of glue listed here will each produce a wood to wood bond that is more than strong enough and more than durable enough under normal circumstances.
Beginners always want to know things like “what is the best” or “the strongest” when those are factors that don’t really apply for joining a material as varied as wood. Better to know “what’s the best glue for this application” or “how strong does this joint need to be”.

Epoxy

Epoxy has the benefit of forming a bond to the wood in any orientation – face-grain, end-grain, whatever. Its strength comes from its adhesion to the material and its own strength. While yellow wood glue does dry fairly hard, epoxy is a much stronger material after it has fully set. And while setting, it doesn’t shrink, so it will bridge gaps well. It doesn’t have to be clamped and is not easily absorbed into the wood. It generally contains no water and most types produce a joint that is very water resistant.

On the downside, it is expensive and has a limited shelf life. It’s also in two parts and needs to be mixed and used within a fixed amount of time. Unlike yellow wood glue, you need some type of solvent other than water to clean it up.

Polyurethane Construction Adhesive

Like epoxy, polyurethane construction adhesive will bond equally to the wood in any orientation. It also relies on adhesion to the base material and it’s own strength to form the bond and doesn’t need to be clamped. And while it is not as hard as epoxy, it is very resilient and durable. It doesn’t contains water and doesn’t shrink, so it has excellent gap filling properties. It is fairly inexpensive and has a shelf life of longer than a year. Again like epoxy, it produces a water resistant joint and the strength doesn’t fade over time. Another plus is that it will set without any reduction in strength at lower temperatures, so it’s perfect for work in a shop that is not heated up to room temperature.

Drawbacks are it’s usually only available in tubes and cleanup can be even more of a problem than epoxy.

So, where would I use each? My recent wooden bar clamps project is a good example where I used all three:

• I used yellow wood glue to fasten the fixed jaw to the bar since the joint was primarily face-grain to face-grain, and that’s where the joint strength would come from.
• I used epoxy to fasten the maple spring to the ash bottom block. Even though I could have used yellow wood glue for this, I trust the epoxy for smaller glue areas like this that will be subjected to stress over and over.
• I used polyurethane construction adhesive basically on every other glue joint, in particular the bridle joint in the “L” shaped clamp pad. A bridle joint has many gluing surfaces, but none (in this part orientation) were the straight face-grain to face-grain that yellow wood glue works best with. These were cross-grain and butt joints and possibly some small gaps that the poly glue will cope with better than the yellow wood glue. Epoxy would have worked even better, but was a lot less convenient to use. In the end, the joint only needs to be as strong as it needs to be, and the poly glue got it there.

In this video, I demonstrate the strength of yellow wood glue that was left to dry in low temperature: