Sanding biscuit & MT joints

[Jack Ferrell]

I have read that it's best to wait until the glue is thoroughly dry before sanding a biscuit joint. Otherwise a dip in the sanded wood may form. I get the logic of this ok, but I have not read if this is the same for a glued mortise tennon joint. Wouldn't the same result occur? At least in the thinner MT joints? Or are MT joints usually made in thicker wood so it's not an issue?
Thanks.

[David Rose]

Jack, biscuits are designed to swell when wet. Or if not by design, they do it because of the way they are formed. The moisture in most glues will swell them enough to possibly create a raised spot on your work. As they dry, it goes back down. So if you cut the raised spot down before they dry... you can see what happens. Tenons don't normally swell enough to matter or be noticed.

Mortises and tenons are normally stronger where they can be used.

David

[Tim Burke]

Jack,

I have had the swelling/shrinking issue with biscuits. They work as David described, and yes I ended up with the slight, but noticeable oval shaped depressions. In this case it was mitered 45 deg joints of red oak, which I was able to sand out. Best to let the glue dry a day or so.

Tim

[Tom Hintz]

I think it is a good idea to let any joint dry completely before sanding. As mentioned, the introduction of moisture changes the wood slightly and can result in deviations that can be seen later when that moisture evaporates and the wood returns to its "dry" state. With wood being such an inconsistent meidum, these deviations can occur with virtually any type of joint.
Also, trying to sand out joints before the glue is completely dry can disrupte the joints alignment.
I rarely find myself in a big enough rush that leaving a project sit overnight for joinery to set up completely is a problem.

[Mark Pruitt]

Yesterday I glued two panels w/ buscuits, at about 11:00 or so. Then headed to the church, got back home about 1:45-2:00 and took the panels out of the clamps and belt sanded them. Three hours from glue-up to sanding. Species is pine, boards are 3/4 thick. I've not checked them today, but last night they looked fine. (This project is just a clamp rack, so there's no real loss if indentations do appear on the surface. I've read of this phenomenon occuring; here's an opportunity to experiment and see for myself.) If I note any changes tonight I'll give an update.
Mark

[Mark Pruitt]

I've checked my panels tonight, with a light at one end and sighting down from the other end. If there were any indentations they would have been evident in shadows. There were none. So, in this one experiment, I have three hours of allowing the glue in the buscuit slots to set before sanding, and at approximately 30 hours after sanding there are no indentations. FWIW.

Mark