Gluing Joints: It's A Surface Thing
Gluing Joints: It's A Surface Thing

I read an article by Gene Wengert in March of 2012. It has been on my mind ever since and I just knew that I needed to go back to that article and really dive into what Gene had said. At the time he had impressed me so much that now, more than two years later, I wrote him and asked his permission to use ideas from his article. He graciously agreed and so here goes!

In coatings, we preach the need to get the coating onto the wood as soon after the final sanding is completed as is absolutely possible. Obviously, as wood is a material that is susceptible to humidity and temperature change, one can see the logic in that concept. But I think we all downplay that and we often tend to do more than to “bend the rules.” I think that we are all guilty of blatantly ignoring this issue at one point or another.

In my shop, my practice was to order doors and then build the cases myself. I would deliberately schedule the doors to come into the shop before the cases were done so that I could go right to fitting the doors as soon as the cases were done. That often meant that the doors were sitting around for a week…or more. As production goes, they often had been finish sanded two week…or more before they were stained and sealed.

Gene Wengert, aka The Wood Doctor, is all about moisture content. So it comes as no surprise that he would write an article entitled “Gluing tips: How to make strong joints.” He goes on to list 15 ways to make stronger glue joints. Well, as a cabinetmaker, I’ll walk out on a limb, take one for the team, and say that we should all read that article. There has to be SOMETHING in that article of value to every woodworker…even if we don’t finish our own cabinets.

But what really struck me was his help to me to visualize how moisture, together with the milling and sanding of the wood, create new means for moisture in the air to affect the wood. That is, once we start subjecting the raw material to those milling processes. It makes a difference!

It also got me thinking that if the performance of wood glue, which I considered pretty foolproof, can be affected by the surface it is applied to, then certainly a wood finish, which is less foolproof and way more temperamental, can’t help but be subject to how the wood surface reacts to the milling process and whatever comes thereafter until the finish is applied.

I know that was a long, tough sentence to digest. But think about it. Those words are at the center of this discussion.


There are two types of adhesion that occur in coatings application. One is a chemical bond and the other is a mechanical bond.

A mechanical bond is one that occurs as the result of sanding. A chemical bond is one that occurs when the new coat re-melts the previous coat allowing both to intermingle and become one. That type of bond occurs only inter-coat.

Equally important is the need for a strong bond between the coating and the raw wood. Over the years much has been said about things that degrade or cause the coating to not bond to the wood or to let go from the wood. Pitch pop in fir is one. Another is the application of finishes to oily woods such as teak. A third and probably the most common would be the over sanding of hard woods such as maple. Doing so can create a surface so slick and/or impervious that the coating has nothing to attach to.

There are ways to deal with this as a prophylactic measure. A “sticky” sealer can be applied to aid the bond between the raw wood and the coating. In the case of oily woods, it is recommended that a wipe-down with lacquer thinner be done to remove surface oils. And again, don’t over sand. Give the coating some “tooth” to hold on to. In the case of maple, I like to sand no finer than 120P.

We have no such things to aid glue’s bond to wood. Other than the lacquer thinner wipe-down, wood glue is on its own to bond to the wood and create a bond that “can be” 50% stronger than the wood itself. That bond is the result of a mechanical action (planing, sawing, and/or sanding of the glue joint surface) before the glue is applied. But Gene says that the longer the glue joint surface is exposed to the air prior to application of the glue and the clamp, the more likely it is that moisture will affect that surface and diminish the bonding qualities of the joint. Gene reports that after a week, the affect is measureable.

Therefore, I would say that my wait of a week or more before applying finish to my doors and cases causes a measureable decline in the ability of my finish system to attach itself to the wood. Gene clarified for me that the issues are microscopic. They occur at the cellular level in the wood. Freshly milled lumber should be flat and straight. However, the longer that the newly milled surface is exposed to the air (Gene says that a dusty atmosphere is even worse) the more that the wood cells will move and change shape to the extent that surface pores swelling up will result in warpage and will affect the bond between surfaces. And, just as importantly, that swelling will reduce the surface’s ability to accept the glue and create the best possible bond.

Gene states that it only takes a few thousandths of movement and it only takes a few days for the glue joint to become inferior to one that was milled and then quickly glued and clamped. The finisher also needs to be concerned with those swelling surface cells reacting with the moisture in the air to make the surface less willing to accept his finish. '

I have gone on for 1000 words now and it occurs to me that I have made a heck of a case for using water-based stains and finishes. Specifically, using these materials on a project that has not been quickly built and finished, may have some plusses to it. But that is a subject for another day.

Until then…Spray on!


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