Biscuit Depression?

This week’s question comes from Jim. He writes:

“I just finished watching all your videos and I have a question about the table top you did in Episode 11. A little while back I saw a episode of New Yankee Workshop where Norm said he stopped using biscuits in table tops because they found that later, depressions showed up in the tops from the biscuit slots. Have you seen this or heard of it happening before?? Oh, and love the videos and all of the site. And now I’m on Lumberjocks as well…thanks to you. Keep on keeping on :>)”

And here was my reply:

Hey Jim. I have heard Norm say the same thing but I have never actually observed it in my own work. As I understand it, the theory goes like this. Once the biscuit joint is slathered with glue and clamped, you essentially create a nearly air-tight pocket. As a result, the glue takes a longer time to cure and the wood fibers around the biscuit swell due to moisture uptake, creating a slight bump where the biscuit is located. At this point in a project, most people will flatten and sand the surface in preparation for finishing. As the glue cures completely over the next few days and the moisture evaporates, the wood fibers around the biscuit compress again and create a sunken area that is visible from the top surface of the table. Obviously this can be quite an eyesore, especially if you used a lot of them as part of the glueup.

While I haven’t ever seen this in my own work, I do pass this warning on to folks whenever they ask me about biscuits. I really only use biscuits for alignment purposes anymore and when I do, I give the workpiece at least 24 hrs to cure before sanding. This is good for the biscuits as well as the glue joint itself where this same sort of depression can occur.

I want to hear from you, the readers, about your experiences with biscuit depressions. Are you a victim? Tell us your story!