You can greatly increase the strength of a woodworking joint by “sizing the joint” or employing “glue size.” But what the heck do those terms mean? And how does the process work?
The word “size” as it relates to an adhesive is a synonym for “washcoat” – a thinned version of the adhesive you are using. “Sizing” with a glue has a long history in our language, and it is employed in a variety of ways – fine art painting, house painting, wallpaper and other mechanical arts.
Basically, to “size” a joint means to apply a thin, preliminary coat of glue to a surface.
Why would anyone do this in woodworking?
I know you’ve been told this a thousand times, but wood is like a bundle of straws. The end grain represents the open ends of the straws. So when you put a liquid on the end of a straw, what happens? It gets sucked into the straw. When you put a liquid on the outside of a straw what happens? If that liquid is glue, then the straws get bound together.
This straw-like property is why end-grain joints in wood are weak. The straws suck the glue away from the joint, starving it of adhesive.
But what if we could plug these straws so they couldn’t suck the glue away from the joint?
And that’s what “glue size” does. The thin coat gets sucked into the end grain. After a couple minutes it begins to set up, which plugs up the straws. Then you apply a full coat of glue, and it cannot be sucked away from the joint – thanks to the plugs. So the joint becomes stronger as a result.
I’ve been experimenting with glue size for more than a decade. At first I thinned glue with water to create my “size.” That seemed logical. It turned out to be unnecessary. Simply paint a thin coat of glue on the end grain and the process works perfectly. Yellow, white and brown glues are all about 40 percent water already and are easily sucked into end grain without any thinning.
After I paint on the “sizing coat,” I wait a minute or two (or longer). The end grain will become dry as it pulls the adhesive in. That’s when I apply a full coat of glue on the end grain and on all face-grain surfaces of the joint.
It takes only an additional minute or two to apply a sizing coat during assembly. In fact, it can add exactly zero time to a glue-up if you simply start by painting all the end-grain surfaces first. Then paint the face-grain surfaces. Then paint the end grain again.
Does it work? The scientists at Franklin International have studied sizing and compared the strength of joints with and without it. As far as I know, they haven’t published a technical paper on this topic. But in interviews they contend that sizing the joint significantly improves its strength. After 23 years of working with the technologists at Franklin, I can say one thing for sure: They’ve never lied to me or led me astray.
Bottom line: Doesn’t hurt. Might help.
— Christopher Schwarz