How to fix an already glued up 2 2x12 fir boards that are cupping?

[Jenny Labendz]

Long time reader, first time poster!
I glued up a desktop I am making made up of 2 boards of 2x12 fir, 61" long. Clamped and glued for 2 days. Unclamped and noticed it's cupped. The side flared up about an inch or half inch from the middle (glued up) seam. I can't plane it. Here is what I am trying now and I need some input. I took some 2x4s and put them under and over each end and clamped it up super strong. With the clamps on it's almost flat. So 2 questions:
1) Will this work?
2) If so, how long should I leave the clamps on?
3) If not, what should I do?
Thanks!

[Steve Jenkins]

Sounds like the edges may not have been square. I’d rip the joint apart the rejoint the edges and reglue. When you clamp alternate clamps above and below the panel checking for flat as you go. Depending on the kind of clamps it’s possible to put a pretty serious bow into the panel if they are on one side.

[Mel Fulks]

Certainly ,check what Steve suggests. But if that does not work ,and if the overall cup is consistant ,(not crimped), I
would place it bow up ,AKA stadium style with
a little weight on it for 24 hours. Close off the open ends with cardboard, no need to do a lot of taping ,it's just to stop
constant air flow. If it does not improve ,keep same set-up and put water on the "dome" with damp cloth . The science
behind the method is the water will make the cells expand , get crowded and make all of them permanently oval (skinny
in the right direction) .
Then, when dry, they pull the top flat.
But you might be able to avoid all that with just a bunch of screws when installing the top. Be careful ,this would
be press and judge ,and work carefully stuff.

[Frank Pratt]

As Steve suggested, the edges may have not been jointed square. I think it's more likely that the wood is just moving.

You have a number of things working against you here. Construction lumber that's kiln dried is still nowhere near dry enough to make furniture out of. In just days it can go from nice & straight to looking like a pretzel. So if you do use it, it should sit in your shop for a couple of months to finish drying.

Another issue is that almost every stick of framing lumber contains pith (wood from the center of the tree), which is very weak & unstable. Even if it's dried to equilibrium with the environment & milled true, it will move a lot with seasonal humidity changes. You have the right idea buying 2x12s, but what you should do is rip the pith out & glue them back up into a slab. That will make for a much more stable top.

Clamping it straight may lessen the cupping, but for the most part, that will be temporary. The best course of action is to rip it apart (and cutting out the pith), add more wood to make up the loss, and reglue.

Fir is one of the harder of the soft woods, but like all soft woods, there is a great deal of difference between the winter & summer growth. Even in fir, the lighter summer rings are still very soft, even though the darker rings are quite hard. This presents a problem with a desk or table top in that those soft parts will dent easily. Don't compose any secret messages with just a piece of paper between pen & top, because anyone will be able to read it engraved in the wood. Use of a blotter will solve that.

How are you planning on finishing it? Staining softwood never yields a good result because it absorbs the stain so unevenly. If you must color the wood, spray on a very dilute dye solution in successive coats until you get the level of coloring you want. Follow that with several coats of polyurethane for protection.

Sorry for the long post, and apologize if I'm over explaining stuff that you already know. Good luck with it & post some pics when it's done.

[David Stone (CT)]

1) Will this work?

Probably not, but no harm in trying. ...Your description of the stock as "2x12 fir" suggests you're working with construction lumber, which likely is at least part of the issue, since low cost is more important than predictable stability for its intended market. Nothing wrong with making stuff, even furniture, from 2x stock if it works visuality and functionally. But it would be the rare 2 x 12 that arrives and stays flat and twist-free enough for furniture use; it needs to be allowed to sit around for a while, dry out some more and do its thing (most likely, distort in some fashion), then be face jointed and planed to yield true stock, just like rough-sawn hardwood.

My advice would be to let it sit as is for a day or two, then come back and check on things. If it's still cupped--as I'd guess it will be--leave it be, unclamped, and give it another week or two to see what, if anything, more happens, good, bad or indifferent. If when you return, the woodworking gods have smiled on you and the cup is diminished to the point where you can live with it, or pull it out when the top's attached to the apron or whatever is underneath, great. Otherwise, rip it apart at the seam and try again, preferably after milling each board as flat as you can.

[Mel Fulks]

David, that is a good thought. I had forgotten that the fir we used was sold to us by one local vender who bought it and
then had it kiln dried by supplier. But it was all dressed to nominal sizes to save money on getting shipped to East coast.
It was a popular product well known to all of the mill work shops. Probably still is.

[Jenny Labendz]

Thank you all! I am probably in over my head because my "shop" is a combination of a studio down the street I pay $18 an hour to use... and the main room of my 2 bedroom apartment I share with my husband and kids. So waiting a long time and going back and forth to the studio with this giant slab is not really an option. I think the wood cupped because I left it to glue in the studio with a window open... during a snow storm. My apartment has central air and is much more consistent. So I guess I will wait a day or so to see if the clamping worked and it doesn't just pop right back. I am planning on staining it and then affixing it to the aprons with tabletop expansion brackets (stretcher plates). Will those screws hold it down? (I learned online that people say these brackets allow for movement. Bulk Hardware BH00122 Cranked Square Stretcher Plate Bracket Brace, 38 mm Square - Pack of 10 https://www.amazon.com/dp/B007SLYOBA/ref=cm_sw_r_cp_api_glc_fabc_mex3Fb23DCZWJ?_encodin g=UTF8&psc=1)

[Jenny Labendz]

Hahah I love the secret messages comment! Thanks for the tip. I know because I made mini desks for my kids out of pine and they're all full of pencil indentations. I had planned on a blotter.

[Jenny Labendz]

This makes me hopeful. If I attach to the frame and aprons with expansion brackets / stretcher plates do you think that will hold the cupping down?

[andy bessette]

...I’d rip the joint apart the rejoint the edges and reglue...

This is what I'd do.

[Tom Bender]

Jenny it will take a little readjustment of your plan but you will get a very functional result if you scrap that top and change to a plywood top. Buy the best plywood your budget will allow. The big box store will saw it to exact size. Two layers, 3/4" + 3/4" will get you the same thickness and the greater weight will improve the feel of your desk. You will have to decide on edge treatment. You can screw the first sheet to the apron and just lightly glue the second top in place. You can probably finish the work in your home which should help with the cost.

[Frank Pratt]

The big box store will saw it to exact size.

Whoa there Tom, I've never seen a big box store that will cut anything to exact size. If they do, then its cause there just happened to be an operator that was extra detail orientated. They even have signs warning the cuts are not exact.

[Thomas McCurnin]

I'm not understanding how much the cup is, or why it cannot be flattened with a bench plane or a wide belt sanding machine.

[Frank Pratt]

I'm not understanding how much the cup is, or why it cannot be flattened with a bench plane or a wide belt sanding machine.

It's in the OP: "The side flared up about an inch or half inch from the middle (glued up) seam." Too much to plane off.

[Tom Bender]

If she asks nicely I'm sure they will get it within 1/16" which is plenty close enough.

[Will Blick]

or cut both sheets simultaneously...
my problem with Borg cuts, the blades are so dull, the ply gets horrible tear out.
agreed, Fir is impossible to keep straight, a mix of poor drying, plainsawn, etc.
best chance, if u find the closest to riftsawn Fir, makes a huge difference..

[Mel Fulks]

Frank, good job on that post, and I'm sure Op appreciates it. Alas, I can find nothing amiss in that missive to the Miss ,
or Mizis

[Doug Dawson]

Whoa there Tom, I've never seen a big box store that will cut anything to exact size. If they do, then its cause there just happened to be an operator that was extra detail orientated. They even have signs warning the cuts are not exact.

Table tops do not need to be an exact size, usually. You take what they give you, and you’re gonna _like_ it. :^) Or else buy a track saw...