How Much Glue?

This question comes from Adam who asks:

I have been at this hobby affectionately referred to as woodworking for about a year and a half now, and I’ve made some pretty decent projects. No substantial furniture, but some nice smaller things. I was in the process of making my most recent project, a box to hold pictures and sit on a coffee table, and I had an epiphany. Every project I have done has had the same problem. GLUE!!! I think I have been making the rookie mistake for WAY too long and thought I would see if you had any input on it. I ALWAYS use entirely too much in my joints. I use blue tape and allow it to skin over and scrape it away, but it is still normally a pretty big mess. I’m always scared that I’m not getting enough glue in the joints when I try to use less, so then I worry myself until I put more. So as a request, could you include a couple of close-ups of your next glue up so that I can get an idea of how much is the right amount? I know as of now that I am using too much, I just don’t want to use too little and have my projects fall apart. I’ve heard time and time again that a thin film is plenty on the joints and I realize that squeeze out will occur”¦but not as much as I am experiencing. Also, the word “thin” is relative. What I put on is a “thin” coat”¦until it squeezes out and runs all over my blue tape. Lol thanks for any help you can throw my way.

And here was my reply:

squeezeoutHey Adam. In the Steamer Trunk Pt. 3, I believe I showed a good shot of a rabbet joint being glued up, and you can clearly see the squeeze out. If I recall, that joint was a little wetter than I usually like. But it was a pre-finished piece so the squeeze out was easy enough to clean. Now as a general rule of thumb, as long as each adjoining piece is coated completely, you have enough glue on the joint. You are right in that all you need is a thin film. And from what I understand, the thinner the film, the stronger the joint.

Now let’s talk about how thin the coating needs to be. Think of painting a wall. You don’t slop the paint on real heavy, right? Instead, you roll it out into one smooth continuous thin layer. And that’s what you should do with your glue joints. In general, you want to make sure each part of the joint has glue from edge to edge. And if you want to add a smidge more for good luck, go ahead.

butterbagelOh and here’s another way to think of it. I just love food analogies! Let’s talk bagels: butter vs cream cheese. When it comes to butter, most folks like a nice thin coating of butter from edge to edge. And with cream cheese, people tend to like a more generous helping. Some crazy people actually like so much cream cheese that its more like having a bagel with their cream cheese instead of the other way around. (I am qualified to speak on this topic because I spent a full year working at a My Favorite Muffin in Princeton, NJ). So glue should be spread more like butter, instead of like a schmear of cream cheese. Now I’m hungry! Lunch anyone?

Category: Techniques

Comments

  1. I think it takes a certain number of projects with major squeeze out before you truly begin to appreciate the “proper” amount of glue required for a joint. It is true that glue molecules bond better to wood molecules as oppose to other glue molecules. So in a perfect world, your glue joint would be one molecule thick, evenly across the joint. However, that’s not practical since it is impossible to cut joints that are perfectly flat and mated with no voids or gaps. But that little fact does change your perspective – the best way to minimize glue (and subsequent squeeze out) is to cut good joinery.

    You also want to consider where the joints strength comes from and focus on those parts. For example it turns out most of the strength from a mortise and tenon joint comes from the tenon face cheeks, not the edge cheek nor the shoulders. So if you are brave enough, and your tenons fit well enough, you can just glue the faces (and mating parts of the mortise) and your joint will be just as strong. Since the end of the tenon adds no strength either, chopping your mortises slightly deeper creates a nice glue reservoir. The Schwarz wrote a great article on the science of the M&T in the latest Woodworking magazine that’s definitely worth a read.

  2. Jeremy Kriewaldt July 7, 2009

    One issue that rarely seems to be raised is one that I only discovered recently when reading J E Gordon’s two books The New Science of Strong Materials and Structures (both of which I would highly recommend – although they deal with technical engineering science stuff, Gordon writes in a simple way that explains his concepts succinctly and clearly; and with minimal mathematics!). That is that the bulk of the sticking that an adhesive line does is performed at the edges of the joint – in fact the glue in the middle of the joint is only a passenger. Under tension and shear the edges are where the stresses are greates and if the edge join breaks then the joint will fail totally because the glue in the centre of the joint will then be under a shearing strain which no glues are good at withstanding.

    So the lesson from this is that the important thing is to ensure that the parts that will become edges of the joint need to have enough glue to make a strong adhesive bond. That’s why a little squeezeout all along the joint is a pretty good thing – it shows that the glue will make an adhesive bond all along the edge of the joint. Blobby squeeze out shows that there may not be a consistent bond along the whole edge of the joint – ie the glue line at the edge may be like this – – – – rather than this ____________.

    The ideal way to achieve this would be a fine and constant line of glue laid down in a line by a syringe on the parts that will form the edge joints. There will be minimal squeeze out and any ‘surplus glue’ will go inside the joint where it won’t be seen (it won’t help there but it won’t hurt either. Since we can’t do that, a coat that goes all the way to the edge is the best practicalway of achieving the strongest bond.

    So spread glue like butter, but make sure that whatever you do, the butter goes right out to the crust. The worst thing to do is to put a blob of glue like cream cheese right in the middle of the bagel!

  3. Zach July 8, 2009

    The bagel analogy makes perfect sense.

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