Clamping to correct cupping in thin boards?

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Forum topic by Wiley posted 09-09-2010 10:44 PM 4227 views 0 times favorited 8 replies Add to Favorites Watch
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71 posts in 3059 days

09-09-2010 10:44 PM

Topic tags/keywords: cupping thin stock question

I do most of my work with 1/4” and thinner wood, which means that it’s not exactly unusual for wood to cup between resawing and actually making the project. It’s not usually too bad, but one particular piece of oak was cupped enough to interfere with cutting the joinery. I’m going away for the weekend, and I thought I’d leave it clamped to a table while I was gone, and hope that it stayed flat when I unclamped it. Once it’s in the piece, it’ll be held in place by other wood and cupping won’t be an issue, I just need it to stay flat long enough to cut some rabbets with my router table. Is clamping like this likely to work, or should I just give up and find a new piece of oak? Is there anything I can do to make it more likely to work?

-- "When you lose the power to laugh, you lose the power to think straight" - Inherit the Wind

8 replies so far

View gerrym526's profile


274 posts in 3836 days

#1 posted 09-09-2010 11:05 PM

Wood that cups is just releasing its internal pressure-what you started to do when you resawed it. No matter how hard you clamp it, or glue it, you still haven’t released the pressure. What has worked for me in the past is starting with a wider board, and ripping it into one or more pieces, see if the narrow pieces have eliminated the cup, then gluing them back together and ripping to the right width. Lots of work, but has been a solution I’ve used in the past. At the end of the day, you might be better off starting with a new piece of wood.

-- Gerry

View Gregn's profile


1642 posts in 3011 days

#2 posted 09-09-2010 11:12 PM

You probably had a humidity change, so I would think put a weight on it would probably be better than clamping it, allowing the wood to move freely to relax and lay flat. When you come back after the weekend, you should see a change and should be flat depending on what your humidity is there.

-- I don't make mistakes, I have great learning lessons, Greg

View William's profile


9949 posts in 2870 days

#3 posted 09-09-2010 11:58 PM

I’m not sure how to get the cup out of this particular board. For future reference though, I thickness plane a lot of my 1/4” material beforehand too. I stack these 1/4” material in stacks with several much thicker boards on top of that stack. The higher the stack of thin stock, the more weight I put on top of the stack. So far, with an axception here and there, this has kept my wood from cupping or warping.


View rkevins's profile


78 posts in 2958 days

#4 posted 09-10-2010 01:24 AM

Thy a search for a product named “Wood-Loc” Creative Woodworks and Crafts had ads for it back a few years ago, If Sheila sees this mabie she can help

View Gene Howe's profile

Gene Howe

10543 posts in 3456 days

#5 posted 09-10-2010 02:23 AM

Probably too much trouble for a small piece but I use an unorthodox method involving Downy and very hot water.
A lid full of downy in a gallon of boiling water poured over the wood will allow it to bend back flat, then clamp or weight it. I use this in a piece of 4” DWV to bend 1X1 oak. It’s like spaghetti after a few hours.

-- Gene 'The true soldier fights not because he hates what is in front of him, but because he loves what is behind him.' G. K. Chesterton

View Keith Fenton's profile

Keith Fenton

328 posts in 2948 days

#6 posted 09-10-2010 03:39 AM

Clamping or weighing down the wood for awhile may lessen the cup some at least temporarily. I don’t know if it will be flattened enough to work with. We do as William said and weigh down our thin wood and it does a great job at preventing the cupping and warping in the first place.

I mentioned it to Sheila but she doesn’t recall hearing about Wood-Loc.

-- Scroll saw patterns @

View Gene Howe's profile

Gene Howe

10543 posts in 3456 days

#7 posted 09-10-2010 04:35 AM

From what I’ve read in a year 2000 publication, Wood-Loc is a PEG like product that displaces moisture in wood.
One comment was “The unwarping of wood is pretty remarkable.”.
On another site, a fellow told of un-cupping a 3/4 board.
But, In a search, I couldn’t find any site that sold it any longer.
Posssibly, PEG might work.

-- Gene 'The true soldier fights not because he hates what is in front of him, but because he loves what is behind him.' G. K. Chesterton

View supervato's profile


153 posts in 2957 days

#8 posted 09-10-2010 06:41 AM

you have to cut the cup out or start with a new piece of wood. no matter how you clamp it it will coe back, next option is to flat plane the cup out if you have a jointer. of course do this when the board is at a 1/4 inch thick then after flat planed use a thickness planer and take it down to final thickness. in other words let it stress first then machine it.

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