Q: I am having a problem with checking in our veneer. The problem seems to come in batches of veneer, while other batches are perfect (same species and supplier in all cases). When we have the problem, we might see a few checks before finishing, but most of them develop after finishing...a few hours to a few days. When we have the problem, it might be as many as 20 percent of the pieces. Can you shed some light on this and suggest what we can look at?
A: In almost all cases, checking in veneer occurs during the manufacturing (including drying) of the veneer. Once the veneer is dried, it is twice as strong as when it was green and so it is unlikely that forces can develop that will initiate new cracks. So, the cracks you are seeing are old cracks that are opening after finishing.
The cracks can develop when the veneer is cut. They can develop when the veneer is dried. In both cases, there are specific manufacturing procedures to avoid such cracks. After drying, it is common to find that the cracks are tightly closed and are nearly impossible to locate. This is especially true if the humidity in shipment and storage is 50 percent RH or higher; this humidity will result in the checks swelling closed. Likewise, when a water-based adhesive is used, the moisture will swell the cracks closed. When the moisture from high RH or from glue leaves the wood, such as when heated in the finishing process or when stored in a dry location, the wood veneer will shrink, creating stress. This stress can easily open existing checks, but the stress level is too low to open new checks in the veneer. Often when the check opens, it cracks the finish. Finish cracks are often very obvious, appearing as white lines.
So what can you do? I suggest that you develop a test where you expose a few pieces of newly received veneer to a little heat and low humidity (90 F and 35 percent RH perhaps). If the cracks are there, they will open. You may have to use good lighting to see them, however.
It is often found that the cracks are only on one side of the veneer. This side is usually called the loose side. If you glue the loose side to your substrate, then the other side (the tight side) will be check free. Just be careful that you do not sand the tight side too deeply, or you will sand into the checks underneath.
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