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My QS Oak Tabletop Cupped....

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Forum topic by BLarge posted 04-25-2013 10:20 PM 1076 views 0 times favorited 14 replies Add to Favorites Watch
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BLarge

147 posts in 1159 days


04-25-2013 10:20 PM

So I let the stock acclimate in my shop for 3 weeks, cut it to rough size, waited another week, the finally milled it down to size the glued it up… The glue up was very good, came out nearly dead flat… I belt sanded the top flat front and back, the applied dye to color the top and bottom… Still flat. I put 5 coats of poly on the top and have been waiting for it to cure, then I will wet sand and rub out to a nice satin luster…. I planned on doing a coat or two of Poly on the underside to to make it complete…

So I am sitting on the couch looking at the table, and it looked cupped ( so the concave is on the underside)... Sure enough I get my straightedge and that top is cupped with and 8th of an inch OB both ends….. What????

Now, I know it isn’t supposed to matter if you finish one side and not the other, but is it possible the Poly on one side caused I to cup like that?? Did I not wait long enough for drying? Does Water base poly cause something like this??

The project looks amazing, the QS top is beautiful, the QS legs visually pleasing… And the birds eye drawer fronts pop like crazy….

To note- the stock acclimated to my shop in the basement, after I was ready to let it cure, I brought it upstairs where it is slightly warmer, and it was up here for two days… I did this because I wanted to work on the drawers and not get dust all over the base and top…

Colorado might be a difficult place to do furniture because it is so dry….

Any thoughts would be greatly appreciated…


14 replies so far

View waho6o9's profile (online now)

waho6o9

5117 posts in 1274 days


#1 posted 04-25-2013 10:26 PM

Maybe the glue up was similar?

View BLarge's profile

BLarge

147 posts in 1159 days


#2 posted 04-25-2013 10:38 PM

Waho,

That appears to be plain sawn in the example, the oak I used was 5/4 quarter sawn…. My understanding is that QS stock should warp and cup less…. Maybe it doesn’t matter??? Frustrating..

The QS is certainly more eye pleasing, I’m my opinion

View Bill White's profile

Bill White

3507 posts in 2657 days


#3 posted 04-25-2013 10:46 PM

How long ago has the wood been originally milled? Do ya have a moisture meter? Will the cup pull down when you attach to the base? If so, I’d let it “rest” some more to see if the bow/cup will settle some.
Don’t panic. Based on your measurements, I’d think the wood will equilibrate to ambient temps and humidity.
The wood we use in our shop is about 8%, and we still wait a while before planing, edging, etc. to final dimensions.
Bill

-- bill@magraphics.us

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waho6o9

5117 posts in 1274 days


#4 posted 04-25-2013 10:47 PM

When you did the glue up, did you alternate the end grain, like one up
one down?

View bondogaposis's profile

bondogaposis

2630 posts in 1048 days


#5 posted 04-25-2013 10:52 PM

I would say that it was not dry enough. I’m guessing your basement has higher humidity than your house. The wood may have adjusted to your basement humidity but that may not be enough. The wood is sealed on top but not on bottom so the bottom is drying out faster than the top, as that is the path of least resistance for moisture to move out of the wood and why it is concave on the bottom. It is not really the fault of the finish but rather that it wasn’t dry enough to begin with. Was the lumber kiln dry? If so, it might have even picked up moisture in your basement. Is your basement heated? Actually Colorado is a great place to do furniture precisely because it is so dry. I would invest in way to measure your shop humidity and compare it to your household humidty, I’m thinking that may be where your problem is. I wouldn’t do anything at this point it may come back down as it equilibrates to household humidity. I hope you allowed for movement of the top in your joinery. Give it a couple of weeks then seal the bottom. I don’t generally finish the underside to the degree that I finish the top but I will seal it and give one coat of finish.

-- Bondo Gaposis

View Charlie's profile

Charlie

1056 posts in 983 days


#6 posted 04-25-2013 11:02 PM

water-based poly?
And you did it on top only?

When I did the waterlox on my walnut island top, I applied it to the bottom, flipped it, and applied it to the top. When it was dry, I flipped it, coated the bottom, flipped and did the top. (5 coats) If you seal a piece of wood on one side so it can no longer breathe, and the other side is still breathing freely, it makes sense to me that it would cup.

View sprucegum's profile

sprucegum

323 posts in 694 days


#7 posted 04-25-2013 11:04 PM

The cupping was most likely caused by the water base poly expanding the grain on top slightly. My old high school shop teacher used to demonstrate this by wetting one side of a wide pine board and putting it next to the heater. you could almost see it move, he would wet the other side the next day and warp it the other way. No reason the water base poly would not do the same thing. It may come back to flat when the curing is complete or perhaps finish the bottom. I always put at least one coat on the bottom of a table top before I install it, I did a small end table this winter from flat sawn ash and finished it with ZAR aqua poly and had no problem with it. The wood however was super dry and I finished both sides the same day..

-- A tube of calk and a gallon of paint will make a carpenter what he ain't

View crank49's profile

crank49

3466 posts in 1668 days


#8 posted 04-25-2013 11:52 PM

With quarter sawn the alternating of grain is a moot point; the grain is all parallel.

I’m sure the water based poly, or any finish other than an oil would have some effect when applied to one side only. If not swelling of the grain, then shrinkage. or maybe some of both. I’m not sure you would need 5 coats, all around, but at least a seal coat on both sides.

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

View RogerInColorado's profile

RogerInColorado

294 posts in 651 days


#9 posted 04-26-2013 01:33 AM

I’m also thinking the poly you used was waterborne. I have no idea if coating the underside at this point will correct the cupping. When you mount the table top to it’s “foundation” you may be able to pull it flat enough for it to pass a visual. Probably worth kicking this one over to the Finishing forum for a more guru intensive audience.

View OggieOglethorpe's profile

OggieOglethorpe

904 posts in 807 days


#10 posted 04-26-2013 02:54 AM

I vote that finishing one side completely first cupped it. The finish expanded one side. If you apply the same coats to the bottom, it may work out.

Alternating smiles is an old wives tale, even with flat sawn stock. It just makes wave shaped tables… ;^)

I’ll normally flip a table for the first 2-3 coats, then finish the top.

View Rick M.'s profile

Rick M.

4208 posts in 1077 days


#11 posted 04-26-2013 03:53 AM

I put 5 coats of poly on the top and have been waiting for it to cure

the concave is on the underside

You sealed the top but the bottom continued to dry. Wood shrinks as it dries so the bottom is now smaller than the top causing it to bow upward. You will need to remove the poly and flatten again. Next time finish both sides at the same time or very close together.

-- |Statistics show that 100% of people bitten by a snake were close to it.|

View Loren's profile

Loren

7746 posts in 2345 days


#12 posted 04-26-2013 04:22 AM

You might shellac the bottom and let the table move for
6 months or a year. It it doesn’t flatten out, take the
top and rip it in half and re-glue.

-- http://lawoodworking.com

View Rick M.'s profile

Rick M.

4208 posts in 1077 days


#13 posted 04-26-2013 05:47 AM

I didn’t alternate boards on any of the tables I built. If they are going to cup I want them all cupping the same way. The only tabletop that cupped was one my wife moved over a heater vent and of course the heat dried out the bottom and the top bowed up. I removed the top and kerfed the underside to get it flat but just sitting around my shop for a week helped quite a bit.

-- |Statistics show that 100% of people bitten by a snake were close to it.|

View bannerpond1's profile

bannerpond1

250 posts in 595 days


#14 posted 04-26-2013 12:03 PM

I’m pretty sure you cupped it by finishing only one side. It is NOT true that it shouldn’t matter. I have had boards cup after planing a new board and leaving it lying flat on the table instead of standing it on edge. When I did so, the cup came out after the board’s humidity evened out. This same cupping will happen if you veneer one side of a board but not the other.

Your QS wood should not have cupped unless your edges were not slam dunk 90 degrees. Always check the jointer table before jointing glue edges. If you are using hand planes, clamp two boards together. Then you will have the angles congruent, even if they’re not 90 degrees. By opening the two boards like a book, with the newly planed edges together, the resulting glue line will equal 180 degrees. Even if the edges are 90.1 and 89.9 degrees to the faces, you’ll still get a flat table top. On a table top, I’d use cauls across the grain while gluing to insure the top doesn’t warp from uneven clamping pressure.

I think I’d rip the boards apart, send the boards through a drum sander to really scuff the finish, and start the glue-up all over again. You gotta finish both sides.

-- --Dale Page

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