What Causes Veneer Checking?

By Bernie Bottens | Posted: 03/08/2013 3:32PM

 

 At the expense of going way out on a limb that someone will surely saw off, I am going to tackle a contentious concept; that of veneer checking. Veneer checking is a common enough occurrence in woodworking. Of course, one has to apply veneer or use veneered panels in order to have this unpleasant event happen to you. I hope that you never face the unpleasantness of this experience. Over the next several weeks we will take a look at veneer checking so that you will be familiar with it.

One of my first trips into Central Oregon when I began working as a wood finishing specialist took me to the dryer part of that wonderful state. West of the Cascades we are, in certain places, in an almost rain forest climate. After crossing the shield of the Coast Range and the Cascade Mountains, however, one finds oneself in a much dryer climate behind the rain shadow of those mountains. There, sagebrush and Junipers grow in a very desert-like expanse. Bernie Bottens how to finish wood My central Oregon friends were in crisis mode. All were standing in a circle with blood in their eye and an “itchy trigger finger” - just waiting for the first to draw his gun. Remember, the first to draw always loses in this scenario.

The confrontation revolves around “who done it.” In this case, the “who” is the one responsible for failure of the veneer panels. But, for us, to discuss this rather complex issue, we first need to ask some questions and then answer them as best we can.

What is veneer checking? How would I recognize it if I saw it? What causes veneer checking? Are there ways to minimize it or keep it from happening? As we answer those questions, please keep in mind the image of all of those product representatives standing in a circle ready to blaze away with their version of what caused the problem and who is responsible. And believe me, it’s always the other guy…not me!

I realize too that this article has the potential of sending me directly under the bus for a very rough ride because no matter what, I am vastly outnumbered in this standoff circle of my own creating. My circle is composed of the same product reps and there is no doubt that they will take exception to what I write. Well, here I go!

What is veneer checking? It appears as cracks in the veneer running with and generally following the grain…not across it. Their appearance may be somewhat minor. They may become more pronounced and eventually may exhibit a rising up along their margins. At that point, those margins may exhibit movement when a fingernail is applied along with some pressure. Obviously, if movement is detected, these cracks also continue up through the finish layer. To some this suggests that the finish layer is responsible. Viewed under magnification, one can see clear down through the wood layer to the substrate. It is not uncommon for the finish layer to be compromised.

My thanks to PianoBob for his posting on Fine Woodworking’s website. PianoBob was restoring an 1896 Steinway that had a very severe case of veneer checking. Please take a look at those two photos. What a bad thing to happen to such a beautiful instrument. But this goes to show us that even the folks at Steinway can have a bad day.

Stress of some kind causes fracturing. First of all, I suspect that when you study what happens to these organisms called trees and you look at the stresses we subject them to when we create veneer out of them, we learn that we are doing a great deal of things that God may never have intended for them. If that were not true, then why would we have come up with terms such as “tight side” and “slack side” to refer to the two faces of the veneer as it relates to how the knife unrolls the material from the log or cant as it is cut?

On the tight side, the blade compresses the wood structures. On the slack side, the material is spread apart. Remember, we are effectively unrolling or peeling the wood. As a means to illustrate, think of what a block plane does to the shavings it removes. Now, take that beautiful curl that we admire when our plane is sharpened to perfection and flatten it out and glue it to a substrate. I think you can see which side is slack and which is tight. Some stretching and compressing is inevitable.

There’s not as much talk about that as there is about the veneer checking, which may appear later in the life of the veneer sheet. But we know that these cracks, as minute as they may be, are there. Furthermore, a lot depends upon which side of the veneer we glue down to the substrate. Slack side up means the cracks are up. Tight side up means that they are down.

This up or down orientation affects us visibly in the finishing stages. The difference between book matched veneer and slip matched veneer exhibits itself in what is known as “barberpoling” or “candystriping”. That’s where every other piece of veneer within the sheet takes stain at a different rate because a different side is facing up. Those darker/lighter stripes are more evident in book matched and less so in slip matched. That’s because in slip matched, the same side of the veneer is always facing up.

Wood, being what it is, is irregular and ever changing. Different areas within the grain structure will be more pliable than others. Then too, there are different methods used to peel the veneer. These methods produce such characteristics as plain sliced, rift cut, or rotary cut veneer. In all of these, the knife attacksthe woodin different ways to yield different grain structure.

Let’s talk a moment about movement in wood. Wood is in a constant state of movement. That movement is wood’s reaction to changes in its environment. Temperature and, certainly, humidity are the things that make it move. Clearly, once we have applied a finish to that veneer we have created a barrier that will greatly reduce movement due to humidity. But it will continue to move none the less.

Almost without exception, we apply veneer to some form of engineered substrate. Call it plywood or particle board or MDF, all of these panels are engineered to be as stable as possible. Particleboard and MDF, in particular, are designed to NOT have a grain direction along which they may expand or contract. Therefore, they will expand or contract equally in every direction when they move.

Plywood, on the other hand, is a stack of veneers with obvious grain direction that have been glued together in an alternating cross-grained manner. There is potential for movement within the plies but then too, the plies are very firmly glued to one another so as to minimize this and to create stability and strength.

The last alternative would be to apply veneer to some form of solid stock substrate. This would not be wise because we all know that solid wood is much less dimensionally stable than engineered panels. That instability (expansion and contraction) is primarily across the grain. Expansion with the grain is there too but the primary movement is going to be cross grain. With the issue of possible movement in the substrate a possibility, failure due to that movement would be quite possible.

Next week we will continue this discussion. I will discuss the different types of veneer that are available on the market, the two most commonly used adhesive types, and then I’ll discuss how the environment affects veneer. I hope that you will stay tuned and that you will be back next week to read along.

Until next time…spray on!


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About the Author

Bernie Bottens

Bernie Bottens (WoodworkingNetwork.com/blogs)writes and teaches on the subject of wood and wood finishing in industrial woodworking. He and his wife, Carol, live in Vancouver, WA. Bernie has been teaching wood finishing to shop owners, shop foremen, spray technicians and finishers all over Oregon, southwest Washington, and northern California for the past 9 years. Prior to that, he owned his own cabinet shop. His shop credentials include apprenticing and becoming a journeyman exhibit builder. Before that he taught in the public schools for 20 years. Bernie is the owner of Kapellmeister Enterprises, Inc. and Kap Coatings Consulting. Reach him at kapenterprises@msn.com.

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