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    Flattening a bowed panel

    I'm looking for some advice on how to flatten an edge glued panel that bowed. The panel is about 25"x25" and is already cut and prefinished. It is for a raised panel design. After finishing, the sides of the panel bowed up about 1/4" on each side. Attached are some pics to help out
    1. What is the best way to flatten this? Counter weight it and let it sit for a little while?
    2. In the 2nd pic you can see where one end of the edge glue has a gap. Could this cause issues later?

    Thanks!
    20171202_222244.jpg
    20171202_222310.jpg

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    Re: Flattening a bowed panel

    This might be only for thin, unfinished panels, but what do you have to lose?

    To get the board to flatten back out you will need to dampen the inside of the cup (the side where the fibers are dryer/tighter). This will cause the fibers to expand and the board will start to flatten. To do this we usually use a damp cloth and evenly wet the one side, you can also use a spray bottle. Leave the board lay wet side down on a workbench overnight. Repeat if it doesn't go completely flat or warps back the other way. Some weight on top to help flatten the board might be necessary for stubborn ones.
    Berta

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    Re: Flattening a bowed panel

    Thanks Berta. That's a good idea, but I don't think it will work here. The inside of the cup is on the side that's already stained and finished so water shouldnt affect it

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    Re: Flattening a bowed panel

    Nate
    Can you get the panel to fit in the frame? I have panels that are not flat in many of the raised panel projects I build. As soon as you make the panel/field, get it in a frame. Do not let it sit around if its plain cut stock.

    In the future, finish the edges that go in the grooves of your frame. This prevents unfinished wood appearing in the dry months of winter.

    Notice the bottom panel. Dye on the edge working towards a color match. All done before glue up. No surprises when the panel is locked in.




    Always build your frame sticking a bit long. It makes it easy to get it apart when test fitting and tweaking.















    The door frames, panels and case stiles have been dyed to match in the photo above. The lower left panel needs to be taken out and bleached on the right side as it was too orange. Re dye.








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    Re: Flattening a bowed panel

    I did not put it together yet. I still have 1 more panel to make and 1 more stile. Do you think if I assembled it now would help flatten the panel or would it start bowing the two rails on the top and bottom?

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    Re: Flattening a bowed panel

    Try turning the panel over so the concave face is down. Leave it overnight and see if it is flatter. This has worked for me, of course I work in a barn so humidity varies quite a bit.

    Roy G

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    Re: Flattening a bowed panel

    Instead of flattening the panel, just flatten the high spot of the tongue only just enough to get it into the slot. A plane or 80gr sanding block should work. I would do this in the middle of the bowed tongues so that the vertical tongues still match the stile slots. I would think that the small amount the tongues go into the grooves would not have enough pressure to bow the rails.

    The other option would be to relieve the back side of the matching grooves on the rails. As the center area of the end grain is normally where you would glue or tack the panel to keep it centered , a slightly wider groove on the back side shouldn't result in any rattling noise. If this was to be fully free floating, I would consider a spot of glue in the center of this panel. If adjusting the groove width, do it only on the back side so everything on the show face remains the same.

    The above is based on my understanding that the center of the bow is away from the show face. Please disregard if I am wrong.

    Go
    Last edited by Gofor; 12-03-2017 at 01:07 PM.
    Practicing at practical woodworking

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    Re: Flattening a bowed panel

    Quote Originally Posted by njohnston924 View Post
    Thanks Berta. That's a good idea, but I don't think it will work here. The inside of the cup is on the side that's already stained and finished so water shouldnt affect it
    If you can bring it indoors to a lower humidity environment then you can temporarily remove the bowing as it will cause the longer fibers (on the convex side) to shrink a bit and flatten the panel some. Of course, those stresses will return when the board takes on more moisture but it will allow you to at least get things assembled.

    When edge gluing panels it is a good idea to take a look at the end grain before glue-up, if the endgrain has a cup shape then the panel will be prone to cupping with any changes in moisture content. In that case it is often helpful to align your boards such that the cupping alternates, that way the panel takes on an often much less severe wave pattern rather than a single severe cup (especially if using multiple narrow boards). But endgrain will nearly always tell you the story of how the board is going to behave when exposed to changes in humidity.

    Note that the finished side will still gain and lose moisture, but will do so much more slowly than the untreated side. It can also be helpful at times to seal the hidden side as well so that both sides gain and lose moisture more evenly in the future or if this project will be going into an environment that often sees short term spikes in humidity (such as with bathrooms). Finish never inhibits changes in moisture content, it just significantly slows the rate at which moisture is gained and lost. Even otherwise stable quartersawn lumber can temporarily bow and cup if only one side is sealed and the other left unsealed since the unsealed side is subject to more rapid changes in moisture content due to humidity changes.

    When thickness planing lumber to thickness it is good practice to take roughly equal amounts off each side of the board to help reduce cupping as you expose interior wood, that may be at a different moisture content to the outer wood, to your shop's ambient humidity. Do not remove all the necessary thickness at once but rather leave some thickness and then wait a few days to a week before final thicknessing to give the wood some time to equalize to your shop. This way the lumber is more likely to remain flatter during construction provided your shop is not exposed to extremely severe swings in humidity from one day to the next. This will help to reduce the likelihood of cupping between the milling and assembly operations. It also helps to allow your boards to acclimate to your shop for a couple months by moving them into your shop ahead of time if you tend to store your wood supply somewhere other than your shop.

    It is also very helpful if your shop is humidity controlled, such as using a dehumidifier, as this maintains a much more stable humidity level year round which greatly reduces wood movement not only from one day to the next but even one month to the next. As someone who is much slower than most due to disability I would never be able to practically kill and assemble projects in the same day, but with humidity control I can literally mill lumber in one season and assemble in another without any significant wood movement or changes in dimensions that can cause headaches during assembly. Humidity control also means very little time spent dealing with tool and cast iron surface rusting plus the shop is much more comfortable since the air is drier and sweat can evaporate during the warmer months (which also makes air conditioning much more comfortable).

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    Re: Flattening a bowed panel

    Thanks for the tips. I'll try to bend it back by flipping it over and putting some weight on it. In the future I'll take better steps to prevent it using these tips.

    This is for a Murphy bed so the back side will be covered up. The back side is currently unfinished but I'll go ahead and put some poly on it.

    Mark - thanks for the idea about relief cuts on the back side. Hopefully I won'thave to go that route but I'll keep it in mind

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    Re: Flattening a bowed panel

    The first thing I would do is to finish the other side of the panel. Difficult to say what you will have until that is done. I have a headboard to work on when I get home. It has two raised panels I made yesterday. One side has a coat of finish, the other does not. I have even tried to put it together yet. I will probably get two coats of shellac on it prior to dryfit of the headboard. I absolutely hate the look of unfinished edges of panels showing and the uneven moisture pickup of finished/unfinished panels will lead to bowing too.

    Even with my finishing, I often have a bit of bow in panels. As long as the affected piece goes together OK, I proceed with glueup. If the panel was totally flat, it might rattle in the frame. A little warping just keeps it from rattling. My pieces never rattle.

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    Re: Flattening a bowed panel

    Quote Originally Posted by njohnston924 View Post
    <snip>
    This is for a Murphy bed so the back side will be covered up. The back side is currently unfinished but I'll go ahead and put some poly on it.

    <snip>
    Your bowing may be because you only finished one side of the panel. Panels finished on one side only tend to lose or gain moisture more slowly than the unfinished side.

    That said, if you can take the bow out of the panel with light hand pressure, I would finish the panel and slap it in its frame or attach it to its base (as appropriate for the project). Most frames/bases are strong enough to take out a little bow.

    -Mark

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    Re: Flattening a bowed panel

    It might be useful to review where the lumber came from, how it was stored, how you milled and stored the panels, etc. Most cupping is due to unequal drying. This can be the result of several issues.

    1/4" in a 25" wide panel should be ok so long as it slides in without much force.

    If you want to try to fix it, clamp the panel in battens and put under an incandescent light or a fan overnight. You may be pleasantly surprised in the morning :-) [edit: convex side up of course]

    In cases where the cup is too severe, the only solution is to rip the panel down the middle, rejoint and reglue.

    Stick it in there I think you'll be ok. Be sure to put some finish on the other side first.
    Last edited by Rwe2156; 12-06-2017 at 01:40 PM.

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