Thick glue lines

Q: We are edge gluing several strips together into a panel. Occasionally we have a joint failure and when I look at the broken joint, I see plenty of glue, but it just didn't hold together well. There is no wood failure. Can you tell what is going on?

A: It is difficult to be 100 percent certain without actually seeing a sample or two. However, what you are describing would be typical of a failure when the two pieces of wood are not close enough together. That is, you have a glue line that is too thick. Most adhesives used with wood require the wood to be between 0.002 and 0.006 inches apart. At that closeness, I do not think that you would see "plenty of glue."

Thick glue lines can develop for several reasons, with the first two being the most common:

  1. Surfaces that are not flat or true at the time of gluing
  2. Pressure is too low (usually in just a small region)
  3. Glue spread rates are too high
  4. Adhesive is too thick or viscous

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Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.