Biscuits for a table top?

[JeffB]

I did not intend to convey the meaning that gluing biscuits is wrong. I pointed out it was a possible choice to alleviate the problem of biscuit shaped depressions showing up in this situation. It is certainly not the only option.

That is true, however it's easy for "too soon" to sneak up on you. It takes much longer for the moisture to dissipate from all the glue in a biscuit slot than from the area along the joint that would cause the "valley" to which you referred. Many people have run into the problem of biscuit shaped depressions appearing along joints. I just pointed out a simple way to alleviate that.

[Hax Planx]

My guess would be that biscuits aren't the problem so much as excess glue. I've heard that yellow/white glue will cause depressions in the wood if used in excess. It makes sense that soft, weak poplar would be more at risk than most other woods. Norm really pours on the glue when he uses biscuits. I'm using epoxy more and more because of all the headaches with yellow glue.

[Charley]

I've found that using biscuits in a tabletop glue-up can cause problems if you surface the glued up panel too soon. The added moisture of the glue makes the area around the biscuit swell slightly, so if you surface the panel right after the glue sets, there will be a slight depression in the surface in the area of the biscuit after the moisture fully leaves the glue and biscuit. Whenever I'm going to make a table or cabinet top I've now learned to let the panel sit unsurfaced for about a week before continuing with the surfacing. This lets the area shrink and stabilize so when it's surfaced it stays flat afterwards.

If I'm in a hurry and don't want to wait to prep the panel, I avoid water based glues and lean toward epoxies for my glue-ups. Even then it takes at least 24 hours for the biscuit pocket and epoxy to settle to a stable thickness before it's wise to surface. If you rush it, you will likely find depressions in the surface around the biscuit areas when the project is completed.