‘Glue Size’ – What is it? How Does it Work?

ADB-15-size-end-grain-IMG_0501

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

About Lost Art Press

Publisher of woodworking books and videos specializing in hand tool techniques.
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21 Responses to ‘Glue Size’ – What is it? How Does it Work?

  1. Steve Vlahos says:

    Great tip. I appreciate the thoughtful explanation.

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  2. Agreed, wholeheartedly. Also, coating both pieces to be glued and giving the parts a minute or two before putting them together helps strengthen the glue bond in those situations.

    Liked by 1 person

  3. Bill Edwards says:

    That’s about the size of it….

    Liked by 1 person

  4. Keith says:

    I have found that their much more thick versions, Quick and Thick, and No-Run, No-Drip glues work extremely well on short grain like miters and butt joints. Just another option.

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  5. Rachael Boyd says:

    I would like to add, that if you let yellow,white glue dry completely like a hour or so then try to glue up your joint there will be very little strength to it . As yellow and white glues don’t stick to themselves well. It will work well with hide glue cause hide glue will self emulsify. adding new hide glue to a 100 year old joint that dried out and came apart will renew the joint to full strength.

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  6. Hugo says:

    Thank you very much Chris for such a quick a thorough explanation. Will try that. Still a bit surprised that is works as good with PVA than hide glue. Thanks a lot !

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  7. Dean says:

    I’m sure sizing end grain with glue helps, but I’ve always thought that end grain joints don’t hold worth a poop to begin with.

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  8. johnanthonyleonardo says:

    It would be good if you could persuade your friends at Franklin to publish something.

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  9. Mark Kessler says:

    I have been doing this for 30yrs mostly cuz some old timer (his name was Woody and he was missing 3 fingers) showed me, personally I don’t really think it adds any strength as you are still just gluing end grain and end up with mostly a glue to glue joint, however if you already have a solid joinery method for simplicity lets say a rabbit on a corner that is screwed I think there is value as the joint is not relying on the glue alone and will help keep everything tight and sealed- maybe I just stated the obvious…

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    • tsstahl says:

      A sized end-grain changes the shape of the ‘straws’ as the wood swells. Not only are the straws clogged, there is a smidge more surface area. The next glue coat is locked in place to cure at mostly the same rate as the mated surface (which also benefits from sizing, though not as much).

      If you just slap glue on both sides and move on, the swelling of wood and wicking of the glue takes place under clamp pressure and the mated surfaces are not free to move as much at points of contact. Sizing both sides swells and softens the wood so that when the clamping begins there can be more wood to wood surface area in contact; this is true of any wood joint, end-grain or not, but the subject at hand is end-grain to long grain joints.

      As the glue dries, the wood shrinks again, but now the intertwined surfaces are locked together better mechanically as well as chemically. Ultimately yes, the joint is stronger, but by how much I couldn’t say.

      I really wish I could remember the source for this explanation. I am not a glue chemist, nor did I sleep at a Holiday Inn Express last night.

      Liked by 1 person

  10. Danny Batchelor says:

    Do the same thing with epoxy. Even two sides of long grain. Coat the two sides with straight epoxy then thicken the unused epoxy with filler to a state of ketchup or peanut butter depending on the size of gap that might be possible. Works great! Of course you have a lot more time with epoxy.

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    • Michael says:

      Yes – I do that all the time for outdoor joinery. I believe West Systems recommends that process in their user guide. The only caveat is the adhesive filler can leave a bit of a visible glue line, even on tight joinery. I recently made a dovetailed mailbox post and the glue line is more visible than I would like. But it is a mailbox post 🙂

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  11. ctrega says:

    “Real glue sizing” is done with hot hide glue.
    Coat parts with thinned down hot hide glue and let cure. Use full strength hot hide glue for final assembly. The full strength hot hide glue will re-activate the cured sizing and bond the joint with super stregnth. This can not be duplicated with liquefied hide glues, pva glue or epoxy.

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    • michael says:

      it works fine with PVA as long as the glue is still tacky. Nothing fake about it. I use it for miters. Works great.

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    • tsstahl says:

      “This [reactivate thinned layer] can not be duplicated with…pva glue or epoxy.” Easy enough to agree with.

      ““Real glue sizing” is done with hot hide glue.” Completely disagree. That might have been true in the age before epoxy and PVA; I wasn’t there, so I don’t know. There are more things in heaven and earth, Horatio, than are dreamt of by man. 🙂

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      • ctrega says:

        “Real” hide glue has to be mixed in a pot and cooked. Using it correctly (with sizing), can can create an instant death grip, unlike any other glue.

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  12. Thanks, fun facts

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  13. BLZeebub says:

    I’ve been doing it for years and also discovered that you don’t have to use a thinned glue to size a joint. My experience aligns perfectly with yours. Now, if I could only find a good 5 cent cigar…

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  14. Dave says:

    I’ve been ‘sizing’ with undiluted yellow glue in the manner you described: endgrain, everything else, endgrain again. Thanks for the validation!

    I’m a clinician, and in this era of “evidence based medicine,” it’s been thoroughly beaten into me that I should notice myself practicing from any idiosyncrasy or armchair theory. While I don’t need to be so serious-minded in my woodworking, it’s a stubborn habit of mind at this point – I feel better knowing I my thought process wasn’t completely crazy.

    Liked by 1 person

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