Fix cup in glue up walnut slab

[Nancy Strevelowski]

Well I made a mistake and glued up all with the same grain direction. This is 1.5” walnut. Was flat for a few weeks then garage flooded and humidity went way up and now it’s cupped across the piece. About 3/8” pronounced in the middle.

what can I do to fix?

i have a sliding table saw, 16” jointer, and a 22/44 drum sander.

the piece is in the long grain direction 48” and across 39.5”

would really appreciate thoughts here.

nancy

[Erik Loza]

Honestly, I would start over. Due to how it’s glued up, I don’t think you’ll ever really be able to control movement. What if you ripped out the glue lines and re-layed the boards back up in the correct orientation? Would the loss in overall width be a deal breaker for the project?

Erik

[Nancy Strevelowski]

I could do that but, this was what I was trying to avoid.
many other thoughts? I’ll do it if it’s the only option.

[Jamie Buxton]

What's the intended use for the slab? Will other parts in the furniture do anything to pull the slab flat? For instance, if this is a table top, can you stand having a beam on edge under the middle of the table? The connections from the slab to the beam would need to be sliding, at least out at the ends -- many figure-8 fasteners for instance.

[Matt Day]

I’d cut it apart and redo it. Other methods like leaving it out in the sun or applying water I have found to be temporary changes.

Too bad, but take it as a lesson learned.

[Jamie Buxton]

Before I cut it apart, I'd move it to someplace which doesn't have the high humidity of your flooded shop, and watch what it does. After all, it once was flat. Perhaps if it dries out it will get back toward flat. Waiting and watching costs nothing.

[lowell holmes]

If you cut it apart, you might consider alternating the the top and bottom of adjacent strips.

[Jim Becker]

Before I cut it apart, I'd move it to someplace which doesn't have the high humidity of your flooded shop, and watch what it does. After all, it once was flat. Perhaps if it dries out it will get back toward flat. Waiting and watching costs nothing.

I'm with Jamie on this. You need to let it rest for a bit on stickers so air circulates on both sides and see if it equalizes. Further, when you fasten it to whatever is going to support it, you have some ability there to bring it back to flat. That's not a huge bow and the combination of letting it dry out and getting it mounted may very well fix the issue. I personally do not embrace the "over/under" board arrangement as I always put the best face to the show side of any board I'm using.

[Brian Tymchak]

I'm with Jamie on this. You need to let it rest for a bit on stickers so air circulates on both sides and see if it equalizes. Further, when you fasten it to whatever is going to support it, you have some ability there to bring it back to flat. That's not a huge bow and the combination of letting it dry out and getting it mounted may very well fix the issue. I personally do not embrace the "over/under" board arrangement as I always put the best face to the show side of any board I'm using.

+1. I would add a little weight on stickers on the top and let it set for a couple weeks. Maybe even add a fan on low speed to move the air in the area.

[Richard Coers]

Curious if you had the glue up sitting on a bench or assembly table when the garage flooded. If you did, grain direction wasn't the problem. I learned that mistake decades ago. I had a stack of glued up panels sitting on my bench when we closed down for the weekend. Big weather change over the weekend with high humidity and rain. Came back in on Monday and the panel on top was a bannana. From then on, all panels are set with complete exposure of all surfaces to free air. I'm in the rip all the glue seams and add another board if that makes it too small camp. Lesson learned, and start over.

[Dave Sabo]

I could do that but, this was what I was trying to avoid.
many other thoughts? I’ll do it if it’s the only option.

Why are you trying to avoid this?

[Mel Fulks]

If you don't have any joints opening up on the ends ,then you have done a lot better work than some. No matter how
accurate I think the jointer fence is, I joint one face to fence ,next face out. Much easier than trying to make the fence
stay perfectly square.

[Robert Engel]

I would like to ask how did you store the top? My experience having an unconditioned shop, I find it it best to keep large panels clamped in cauls and covered. Even one night of unrestraint can be a disaster ;-|.

I would clamp it in robust cauls. You may need some spacers to get enough pressure. Let is sit for 3-4 weeks in a well ventilated, but quiet (no fans) corner of the shop. You may have to incrementally tighten the clamps every few days. Or, if possible, bring it inside your house where the humidity is less variable.

That said, I wouldn't be too optimistic. Once cupped, boards tend to "set" and usually don't cooperate too well.

If you are averse to ripping it apart (not as radical as it seems, BTW) another way is to "break its back" by cutting deep grooves in the bottom along lowest points of the cups, then clamp it flat and fill the grooves with epoxy (I've found you usually have to overflex just a bit as there will be some spring back).

Obviously this will not work with exposed ends.

[Dan Hahr]

If it was flat when dry it will go back when dry again. Leave it in a humidity controlled air conditioned space for a couple weeks. It will flatten out on its own.

Dan