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cup top, finish?

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Forum topic by james3one posted 11-02-2012 03:00 PM 707 views 0 times favorited 23 replies Add to Favorites Watch
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james3one

41 posts in 1463 days


11-02-2012 03:00 PM

Topic tags/keywords: question finishing

Have a most unfortunate turn of events. I have a large top on a large pedestal that is cupping. This has caused the top to be unstable, rocking from one side to the other. This is pieced together with four large, 8/4 boards of Poplar. The top was laminated together with the growth rings alternating up and down across the top, like its supposed to. My suspicion is that the seal coat on the bottom isn’t sufficient to keep out the moisture and the five layers on top are. This would allow the bottom to expand and cup towards the top.

My question is this: would adding additional layers of finish to the bottom prevent this kind of movement? Also, will the board equalize(eventually) when the finish is applied?

-- James, Tulsa OK,


23 replies so far

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CharlieM1958

15706 posts in 2909 days


#1 posted 11-02-2012 03:14 PM

I think your assumptions about the cause are correct. However, I would not expect good results from sealing the bottom while the surface is still cupped. Intuition tells me that would just tend to make it stay cupped. (I could be wrong…. I don’t claim to be an expert on this topic at all.)

My thought would be to try to move the table to as dry a location as possible to see if it will flatten out, and then seal the other side thoroughly.

-- Charlie M. "Woodworking - patience = firewood"

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lumberjoe

2842 posts in 939 days


#2 posted 11-02-2012 03:24 PM

I read something recently (can’t remember where, it could have been here) where a similar issue was corrected with a dehumidifier. Get a small one and place it under the table. Cover the whole thing with plastic and let it run (make sure it doesn’t overheat). Apparently that was enough to flatten the table top. Once it is flat again, you can address the seal coat on the underside.

-- www.etsy.com/shop/KandJWoodCrafts

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james3one

41 posts in 1463 days


#3 posted 11-02-2012 03:29 PM

Charlie, that’s kind of what I was thinking. Temps are going up today and I can keep it in my front workroom(as opposed to the wood shop) with the A/C on full to dry things out.

I’ll keep checking it with a straight edge to see how it goes.

-- James, Tulsa OK,

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CharlesNeil

1141 posts in 2561 days


#4 posted 11-02-2012 03:34 PM

I suspect your correct, the bottom has taken on some mositure , the good news is you may be able to salvage it, if it were me, since the bottom has very litte finish on it, I would sand the bottom enough to open it up a little, and then lay it out in the sun bottom up, the object of the game is to let the sun further dry the bottom, making it shrink and reverse the cup, basically get the moisture in balance, if possible. For what ever reason cupping seems to be a big deal these day’s lots of post on it, and lots of debate , somewhere out here we have talked about this. I for one have ALWAYS finished the underside of anything, at least enough to seal it up , and have not had issues, ( knock knock on wood) . Not knowing the finish that is on this, I will share another thing, over the years I have used and seen this happen numerous times, alot of the penetrating oils, that dry slow can also cause it, unless you do the same to the bottom. Keep the wood in balance, I know many who don’t finish the under side and get by with it, its never worked for me personally. The thing with the oils the wood absorbes the oil, its a liquid, and any liquid will swell wood to some degree, then over time the oil finally dries, and as it does the wood will shrink some, thus the cup, I know this sounds a bit far fetched, but I have seen it, a quick story about this, several years ago, I built a large walnut extension table, with 4- 12” leaves. The client insisted I use danish oil, so I did, about 4 or 5 coats as I recall, I did both sides, and it looked great, also did a server for him, when I went to reassemble the server the drawers were overly tight as well as the doors, the finish had dried about 10 days, at this point, but they worked, assembled the table and delivered, about 6 to 8 months later the client calls me and tells me the table no longer fits, I went and checked it out, when the table went together, it had a decisive 3/8 gap in the middle, it was a simple fix as the top was attached via slotted holes in the aprons, so it was loosen and slide it back together,I checked the server, the drawers were almost loose now and the doors fit perfectly. I came back to the shop got a piece of walnut and a piece of cherry , oiled them up well on just one side, left them for a couple of days , when I looked at them they were cupped some to the non oiled side, I did it with growth rings up the other down, didn’t make a difference , I actually forgot about them , not long back I came back across them , they were now cupped in the opposite direction, hummm go figure !

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crank49

3458 posts in 1662 days


#5 posted 11-02-2012 03:35 PM

I’m no expert either. I have been observing the effects of uneven drying as I am in the middle of drying a bunch of cherry.

I think you are correct in your analysis of the problem.

If you have a portable room dehumidifier you could direct it at the bottom of your cupped top. I have used this to de-cup a wide board.

If you have no dehumidifier an ac duct directed at the bottom of the top might help. In any case, you want to increase the air circulation across the bottom of that piece. If it can be excessively dry air, so much the better.

Once it is flat, then you need to look at a more balanced sealing.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

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james3one

41 posts in 1463 days


#6 posted 11-02-2012 03:50 PM

Charles, I just posted the project. I did use an oil finish, and only included the bottom in the final seal coat.

Weather is warm today with lots of sun. I have an assembly table I can put out on the porch to get direct sun. My client decided that she wanted this to be darker, so I’ll strip the wax, sand the bottom and set it out to dry. Assuming the humidity is low enough.

-- James, Tulsa OK,

View CharlesNeil's profile

CharlesNeil

1141 posts in 2561 days


#7 posted 11-02-2012 04:11 PM

James if you can get it in good sunlight, that should do it, the humidity wont be alot of issue, as long as the sun is warming the bottom of the table top, also if you can possible set up a small fan to move a little air over it, will help but do watch it close, sometimes they can move fast, and actualy a little reverse isnt a bad thing, but just a little,

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Cosmicsniper

2199 posts in 1849 days


#8 posted 11-02-2012 05:06 PM

I’m going a different direction with this, especially since you did Sealcoat the bottom.

Looking at the design of that octagon project, it looks like 7 layers of poplar. I assume the top is the only laminated board and the rest are an octagon rim, for lack of better word. So, the top is open to air through the bottom of the pedestal.

If that is the case, my question is how did you attach the top?

-- jay, www.allaboutastro.com

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james3one

41 posts in 1463 days


#9 posted 11-02-2012 05:20 PM

Cosmicsniper: Gravity. The piece is far to heavy to move as a single object. The top sets into the first ring, held in place with shaped blocks. The first set(3) of rings are attached together as are the bottom set of rings. Three pieces total. Had to keep reducing the size of the individual parts to get the weight down to something a bit more manageable.

-- James, Tulsa OK,

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Cosmicsniper

2199 posts in 1849 days


#10 posted 11-02-2012 05:24 PM

Ah. Good. I didn’t want to insult you by saying the “glue” word in that context. :)

In that case, I’d have to follow Charles’ lead on that one.

-- jay, www.allaboutastro.com

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james3one

41 posts in 1463 days


#11 posted 11-02-2012 06:36 PM

Update. its been out in the sun for an hour and the cup has reduced by half. I’ve sanded off the seal coat and will have good sun for at least another 3-4 hours. Actually hoping for the cup to go past even. cupping that direction would actually keep it very stable.

-- James, Tulsa OK,

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CharlesNeil

1141 posts in 2561 days


#12 posted 11-02-2012 06:44 PM

how do you get a wink on this ** thing, he he, it will go just keep an eye on it,

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CharlesNeil

1141 posts in 2561 days


#13 posted 11-02-2012 06:48 PM

once its good to go, try to wrap it up as best you can, a blanket on the finished side would be smart then get it in some plastic, unless you have time to work the finish tonight, you want the blanket as a barrier between the finish and the plastic, some finishes can react with plastic, also if I were you I would skip the other oils and go straight with the Arm R Seal, do a quick wet coat rubbed on, and wiped back, this will not be enough, but we want to get a seal on there and get it dried fast, Arm R Seal will do that

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james3one

41 posts in 1463 days


#14 posted 11-02-2012 06:49 PM

My thanks to Charles and the rest. I’ve gotten lots of good advice on this one.

Keeping an eye on it.

-- James, Tulsa OK,

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james3one

41 posts in 1463 days


#15 posted 11-02-2012 06:56 PM

I was planning to go right to the arm r seal. I know now this will move back. I have some time and will get it to the shape I need. Thanks again.

-- James, Tulsa OK,

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