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Thread: Another way to flatten plywood?

  1. #1
    Join Date
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    Question Another way to flatten plywood?

    I'm building a cabinet for under the extension wing of my TS and one of my pieces of plywood has a pretty good bow in it diagonally from corner to corner. The piece is 27"x30" and is an interior partition separating the router area from the storage area.

    Upon dry fitting the cabinet together I noticed that it throws the whole assembly a bit kittywhampus.

    A search of smc yielded a method of soaking the convex side with water to get it to expand. I may try this tonight.

    Is there a way to get this panel flatter? I thought of making some kerfs in it to relieve the stresses. Has anyone done this or have another suggestion?

    Looks aren't a concern as this is an interior piece and the cabinet is for "go" not "show".
    Kyle in K'zoo
    Screws are kinda like knots, if you can't use the right one, use lots of 'em.
    The greatest tragedy in life is the gruesome murder of a beautiful theory by a brutal gang of facts.

  2. #2
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    Kyle, I've always placed an additional support behind the plywood and then heavily screwed the piece in place starting @ one corner and working to the other. If that didn't work I got another piece of plywood. LOL. Maybe that's why I'm not a master carpenter.

  3. #3
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    I would simply just replace it if its that significant, if its not too bad put it in; after all its just a shop cabinet anyways.

  4. #4
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    I've usually not found bows to present too much of a problem if there is structure behind them or if they are captured in dados (though it can be tricky to get them INTO the dados). Once everything is fastened together it remains solid and in the intended place. The movement doesn't seem to be as strong as in solid wood.


  5. #5

    I would get a new piece of plywood.

    Many projects get ruined because the builder want to save a few bucks or is determined to use a bad piece of wood 'just because'. Not spending any more money for plywood today may not seem like such a good idea tomorrow. If it throws your whole assembly out of whack, doors and drawers probably will not function properly. Any method you use to straighten it out is a gamble at best. I'm not saying it cant be done I'm just saying is the risk worth it?

  6. #6
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    Buy a new piece of plywood.

  7. #7
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    Where do you guys manage to find plywood that doesn't have some warp? Every sheet I've bought in the past year or two had had some, US or Import, Borg or specialty supplier, thick or thin (but especially thin). Even the baltic birch has had it.


  8. #8
    Quote Originally Posted by Matt Meiser View Post
    Where do you guys manage to find plywood that doesn't have some warp? Every sheet I've bought in the past year or two had had some, US or Import, Borg or specialty supplier, thick or thin (but especially thin). Even the baltic birch has had it.

    When I lived in Detroit, I used "All America Plywood" on John R, north of 6 mile, I think.

  9. #9
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    Quote Originally Posted by Tony Bilello View Post
    Many projects get ruined because the builder want to save a few bucks or is determined to use a bad piece of wood 'just because'. Not spending any more money for plywood today may not seem like such a good idea tomorrow. If it throws your whole assembly out of whack, doors and drawers probably will not function properly. Any method you use to straighten it out is a gamble at best. I'm not saying it cant be done I'm just saying is the risk worth it?
    What Tony said. I tried to live with a bowed piece of cherry ply in a cabinet side and it played havoc with the entire project every step of the way. I'd already rejected one piece, from another sheet, as too warped; I should have kept going until I found a suitable piece, for all the trouble it caused me.

  10. #10
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    What you have there is "twist" rather than bow or cup - just splitting hairs

    Your idea of applying water to the convex side to make it expand would actually WORSEN your problem - Wetting the concave side might make it expand enough to take the twist out but what are you gonna do when it dries?

    If it wants to be twisted, it's gonna stay twisted no matter what you do to it. If you want it straight, you'll have to fasten it to something that wants to be straight more than it wants to be twisted. That is, stick it to something stronger.

    Kerfing? With twist, your kerfing would likely have to be diagonal and it'd be a crapshoot. Try it if you have a backup piece of ply ready to step in if you find out the twisted piece was unfixable. Note: I wouldn't try this with my tablesaw, by the way. Circular saw only - and even then it's not really something i'd relish doing.

    Generally speaking - if it wants to be a shape, adding or removing material is the only way to change that shape. Adding water is a really silly concept to me (to ME) because it's gonna dry and go right back to the shape it wants to be. Maybe if you only wanted to fix it temporarily?
    Jason Beam
    Sacramento, CA

  11. #11
    +1 for a new piece. I have given up exchanging my time and effort for poor results just to save a few dollars in material or a bit of labor in re-making a component. I used to try to force things or to "get by" without having to run to the lumber yard 'one more time'. I was always disappointed in the long run and spent more time and money going back and doing it right. YMMV.
    “If liberty means anything at all, it means the right to tell people what they do not want to hear.” -- George Orwell


  12. #12
    Another possibility is to screw a batten to it which will hold it flat. You could probably unscrew the batten after it is in place and the rest of the assembly will hold it flat.

    Of course, that's a cheap guy's approach. The right way to do it would be to buy better ply. I don't always do things the right way.
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  13. #13
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    I'd say get another piece but I would also try to flatten it more using a variety of tricks.

    Clamp a board to it (better if the board is a bit bowed) like you might a caul to pull it a bit beyond flat the opposite way it wants to go. I'd wet both sides and then set it aside to dry for a couple of days and that see what happens.

    The thing with wetting it is you don't know what ply might be pulling the plywood out of shape. Odds are decent it is one of the outer plys but it might not be.

    Wood is surprisingly strong. Check this out:

    I laminated a 1/16" piece of walnut veneer to one side of a sandwich of two 5/8" thick laminated pieces of MDF. The MDF sandwich was about 17" x 48". After pressing for 1 hour, I took it out of the bag to cure for several hours. The sucker bowed up on me a good 1/8" in the middle by the next day. I could easily push it flat with my hand so I wasn't going to worry about it (plus I was adding hardwood edging to it anyway). I let it sit for a couple more days and it settled back quite a bit but still had a slight bow to it. My theory is that the veneer glue added moisture to the veneer, caused it to swell and thus move, and then dried back out again and more or less settled back.

    Anyway, food for thought on trying to flatten your plywood.
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  14. #14
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    When I have a piece of ply that 's not perfectly flat, i place it on a flat surface and force it straight by hand pressure.
    If very little force is required to make it conform and I figure it won't overstress the structure I want to use it in, I use it. If not, I don't. This is "seat of the pants" judgement, and I try to be sure that any error will be on the conservative side.

  15. #15
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    Thanks for the advice!
    Kyle in K'zoo
    Screws are kinda like knots, if you can't use the right one, use lots of 'em.
    The greatest tragedy in life is the gruesome murder of a beautiful theory by a brutal gang of facts.

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