Glue Application: One Face Only, or Both Faces?

      Here's a longish discussion of one worker's preference for applying glue to both pieces he's joining. Is this a good practice, a bad practice, or neither? April 21, 2011

I have just started working with a new employee in the shop/field. He insists on spreading glue onto two edges when gluing up. We typically use Titebond II for most situations. The job we are currently installing consists of many 10' lock mitred columns. They run scribed floor to ceiling and require a speedy glue up. I feel the double glue up is overkill on the glue and could create a possible problem with drying before fully clamped. Just wondering what your techniques are when using yellow glue.

Forum Responses
(Adhesive Forum)
From contributor G:
Try this: yellow glue, edge grain to edge grain, one side, and epoxy two sides. When gluing end grain to end grain (mitered corners, crown) with yellow glue I coat both sides.

From contributor J:
Why two sides for epoxy?

From contributor G:
Most epoxies are of the wetting variety which means it soaks into the wood. When it soaks in it reduces the epoxy on the surface. If this happens to much you will have a dry starved joint. This will be weak and will likely fail. By putting it on both surfaces you will greatly reduce the chance of this happening. Also remember when you use epoxy to not put too much pressure on the joint as this will squeeze out the epoxy and give you a dry joint with the possibility of failure again.

From contributor M:
I tend to be this kind of employee. If it is something I've done and it works, I get irked when someone else tells me to do it differently. Now, that being said, if this guy is slowing you down, try your best to work with him, maybe buy him a couple of glue-bot rollers or something. Make it known that you have done what you are doing successfully for "x" years and the he's costing you man hours. That is, if it's this big of a deal. I donít know how much time he's eating by doing this and it's going to have to be your judgment call. Personally I see both sides of the argument so long as he's not really slowing you down that much.

From Gene Wengert, forum technical advisor:
When the adhesive, such as TB II, is warm or hot (summertime), there is the possibility that it will cure too fast before you can get the pressure on. That is, the moisture in the adhesive will soak into the wood before the pressure is provided. Coating both sides will not stop this pre-curing and in fact it can increase the risk as your open time is longer.

Each adhesive is different. Some have higher solids and some have higher moisture. With higher moisture and less solids, you generally have more open time; TB II is not one of these adhesives in my mind. Of course, the MC of the wood is also a factor - drier wood shortens open time with most adhesives as it soaks up the water faster.

In most cases, increasing the spread rate and coating one side will give you less open time and then the pressure will spread the adhesive to the other side as well as to all surfaces before the adhesive has a chance to pre-cure, developing a uniform, effective thickness of adhesive. There will be some squeeze out in a good joint, meaning you have enough adhesive. Avoid excessive pressure. (For the record, pressure does not squeeze adhesive into the wood cell structure. The holes between cells are too small and the adhesive is too thick to have this happen, except on end grain).

Pressure is to transfer adhesive to the mating surface, spread the adhesive into nooks and crannies, develop a desired thickness of the joint (0.002 to 0.006" in many cases), and to squeeze out any excess. If the moisture has already left the adhesive, then several of these process steps cannot take place. In some cases, I have seen the wood rubbed with a damp rag right before glue spreading in order to wet the surface and thereby extend the open time substantially.

From contributor M:
Gene, when gluing a mitered corner, such as on a jewelry box or on a crown mold corner, some folks like to advocate sizing the joint - gluing both sides and letting them sit, joint to joint, for a minute or three then pull the joint apart and re-glue and leave for the final curing. Does this benefit anything in your opinion?

From Gene Wengert, forum technical advisor:
Gluing end grain in a miter joint is really tough as the open wood structure results in rapid absorption of the water in the glue and the glue itself, so the joint has a high risk of pre-cure and also being starved. By pre-gluing, as you describe, the net effect is that the end grain is sealed somewhat. So, this is a good idea, but only if the adhesive has good strength when bonding to itself. I believe that TB II is not good in this respect, but some of the older adhesives were very good, especially animal glues.

Overall, I would suggest using a hot melt PUR for a miter joint. (Industrial hot melts get much hotter than the home style hot melts). PUR has the advantage that clean up of squeeze out is very easy with no effects on finishing later on.

From contributor G:
I usually coat both surfaces when gluing miters/end to end grains. Helps with the absorption problem, other than that I glue on one edge only.

From Gene Wengert, forum technical advisor:
I was thinking about the original question some more. A glue joint requires so much adhesive (0.005" x width x length plus squeeze out). It is easier to spread all of the adhesive on one edge and then let the pressure spread it to the other side. If you were to actually spread this amount of adhesive on two edges, you would have a lighter coat overall and this would dry faster (double the exposed surface area) giving you less open time (or assembly time). So, to offset this thin coat effect, you might increase the spread rate and that is costly, etc.

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