Q. What is the difference between regular wood adhesives and those that say “exterior?”

A. Here is a general explanation…read the label for complete information.

Interior Adhesives will soften or lose strength (the joint will likely come apart) when exposed to water or heat, especially after an hour or two of exposure.

Essentially, the glue, with water exposure, begins to return to the consistency that it had in the bottle before it was used. We might say that the gluing reaction as the glue cured can be reversed with water or heat.

Exterior Adhesives will not soften when exposed to water or heat for a longer time. The chemical reaction when the glue cures is somewhat, or totally, irreversible.

Many exterior adhesives use heat to cure (usually you apply the heat needed, but epoxy makes its own heat) and/or use a chemical catalyst, such as formaldehyde. (The catalyst, after a reaction is released, so newer adhesives limit the amount of “F” allowed or must reduce it to zero.)

Special note: Either type of adhesive, if properly applied, will create a joint that will be stronger than the wood itself; that is, joints with exterior adhesives are not necessarily stronger than interior if kept dry.

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.


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