Cupped board (glued up)

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Forum topic by Johnny24 posted 04-13-2016 01:29 PM 1469 views 2 times favorited 8 replies Add to Favorites Watch
View Johnny24's profile


8 posts in 799 days

04-13-2016 01:29 PM

Howdy everyone.

New member but a long time lurker.

I glued several strips of wood together (making a dice tray). And the board cupped.

I think, but not sure, the cause of this was after I planned both sides, I placed it down flat on my bench for about a week exposing 1 side to moisture or allowing moisture to leave. Not sure which happened.

I’ve done some research with moisture and wood for the past couple of weeks and have made efforts in making spacers for my lumber and ordered a moisture meter to assist me in my current project and future projects.

Looking for some tips of how to fix this and understanding better what is causing this to cup.

I’ve placed the board under pressure for a couple of weeks and reduced the cupping a few millimeters and put it through my planner and reduced the cup a little more. I don’t/can’t plane it any more.

I’m looking for some correct information regarding on how to further remedy this issue. The info I am finding is conflicting.

I see some people say dry out (in the sun) the cupped side (the concave side) and others say dry out the crowned side (convex side).

Which is it.

From my research via google, the crowned side (convex side), is the side that has a higher moisture content than the cupped side (the concave side).

Is this an accurate statement?

I see people mentioning adding water to the cupped side (concave) and other say add water to the crowned side (convex side).

Not sure which is accurate, but if it is the cupped side that has less moisture content, it would make sense to ass moisture to that side, correct?

I have used my heat gun to blast the crowned side (convex side) to try to remove some extra moisture. It had a little to no effect compared to the weight that I left on it for a while. Perhaps this does not work or I am heating that wrong side?

I’ve recently come across this little how-to

These instructions state to add moisture/steam to the crowned side (the convex side) to help uncup wood. How accurate is that, as stated above, I am under the impression that it is the crowned side that contains the higher moisture content that is causing the cupping.

I’ll find out a bit more when my moisture meter arrives in 7 to 10 days.

thank you for reading.

8 replies so far

View CharlesNeil's profile


2410 posts in 3894 days

#1 posted 04-13-2016 01:56 PM

turn it over , convex side up , put it in the sun a little while, be careful it can reverse quickly …works well

You are correct, leaving one side uncovered and the other exposed will do it everytime. I like to cover my material or better yet store it in plastic bags when not being worked on.

View a1Jim's profile


117115 posts in 3601 days

#2 posted 04-13-2016 01:59 PM

Charles knows what he’s talking about ,go for it.

-- wood crafting & woodworking classes

View a1Jim's profile


117115 posts in 3601 days

#3 posted 04-13-2016 02:01 PM

Welcome to Ljs

-- wood crafting & woodworking classes

View Snipes's profile


177 posts in 2269 days

#4 posted 04-13-2016 02:43 PM

Charles advice is good, I’ve done it many times..

-- if it is to be it is up to me

View JBrow's profile


1361 posts in 944 days

#5 posted 04-14-2016 03:40 AM


Wood fibers swell and shrink when moisture is added or taken away. I do not know for sure, but I believe wood transfers moisture more readily after being freshly milled. The wood fibers on the exposed surface of a panel can give up moisture while moisture remains trapped on the opposite face lying on a work bench. The exposed surface will shrink across it width while the surface resting on the work bench will not. The result is a cup across the width of the panel. The crown will be across the face in contact with the work bench.

On the other hand, if the exposed face absorbs moisture, while the face lying on the workbench absorbs none, the panel will cup, but in this instance, the crown side will be across the exposed face.

In both instances, the crown forms on the face swollen due to greater moisture content. The panel will return to flat when the moisture content of both faces equalizes. This can be done by elevating the panel so that air can circulate freely across both faces. The process can be hastened by either adding moisture to the face opposite the crown or removing moisture from crowned face. When hastening the process by adding moisture to or driving moisture from a face, care is required to ensure that neither too much moisture is added to or removed from the panel. Otherwise the crown could be transferred to the opposite face.

I have removed a crown like you described by clamping the panel across its width in straight cauls, making the panel flat. The crown was down. The clamped cauls squeezed the swollen wood fiber together. I moistened the upward facing surface with water and laid dampened rags over the panel to keep the panel from drying out. I checked progress frequently by measuring the extent of the cup. It disappeared after about 3 hours, returning the panel to flat. I kept it flat by ensuring it was thereafter stickered.

Planing a panel to flatten a moisture caused cup may or may not work. Conceivably enough wood is removed from the crown to eliminate excess moisture, leaving a panel with equalized moisture on the faces. If not, the crown could reappear as moisture content equalizes.

View MNgary's profile


301 posts in 2441 days

#6 posted 04-14-2016 07:00 AM

Did you turn it over and put it in the sun for awhile? How did that work for you?

-- I dream of a world where a duck can cross the road and no one asks why.

View Johnny24's profile


8 posts in 799 days

#7 posted 04-14-2016 01:22 PM

Thanks all for the replies.

Going to be putting it in the sun light today.

First day that is has been warm-ish and with sun for a while.

It’s been cold here in upstate new york. And no sun.

View Johnny24's profile


8 posts in 799 days

#8 posted 04-17-2016 12:10 AM


Panel is 99.9 percent flat. It’s flat as a board. :p

Just a heads up from anyone else who is going to try this trick. My board was a bit thicker than a 1/4 inch, not quite a half inch.

I put the board convex side up on top of an MDF board out in the sun for about 3 or 4 hours.

The board started to cup the other way to a more severe cupping, but not by too much, 3 or 4mm at the center.

Turned it over, left it out for about 2.5 hours. A slight cup started occurring the other way. It was better than the original cupping, but still cupped.

Turned it over, left it out for 30 or 40 minutes and became 99.9 percent flat. Gave it a nice sanding and smooth as ice now. Have not moved on to the next step of my project yet.

It’s been 24 hours and the panel is still flat. Going to let it set for a few days to make sure cupping does not come back.

Personal advice, check your board or panel every 30 minutes or so. This method works a lot faster than I thought it would have. I was expecting this process to take several days to a week or 2.

No damage to the board out side a hair line split where I glue 2 strips together. Not sure how to remedy this issue. It’s about 1/32 thick and runs down about an inch or along the glued edges.

CharlesNeil’s advice gets two thumbs up from me.

I’ll post pics Monday night or Tuesday. Busy weekend studying for Pharmacology and not wood working :(

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