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Cupping table top midway through fininsh

1/11/22       
Tim Member

Website: http://granarywoodshops.com

I have a 116" x 46" black walnut table top; all wood dried to approx MC 8% I stained top and bottom with Minwax walnut stain. Dried 3 days; 2 coats of oil based poly on bottom; then flipped over. I got distracted and was not able to varnish top for 2 weeks....so I have a cupping issue.

I know WHY. In the interest of saving us from going down that rabbit hole....what I really need is HELP with what to do next. Bottom is varnished, top is now stained ready for finish......how can I best get back to flat? Without marring the unfinished top?

Thx in advance for your wisdom.

1/11/22       #2: Cupping table top midway through fi ...
the google

been there, done that.

1. finish the top side as to your finishing schedule and wait for it to flatten out. you can only support the middle or outside edges to help get the top to move towards flat. we did this on a 147" x 60" top recently. it was curled .5" up (dipped .5" in the middle) and is now flat.
2. flood the top side (i'm assuming it is cupping up) with water. this will/can get the top to flatten out semi-immediately. finish from there.

i've done both, but prefer #1 since it is less work.

good luck.

1/11/22       #3: Cupping table top midway through fi ...
gary

Based on your location (same as mine) and the time of the year I assume the unfinished side is concave so moisture needs to be added to it. I have successfully done this by putting the concave side down on the grass outside in the summer. I kept checking it until it was straight and a little more then brought it in. You will probably have to build some type of simple open top box to set the wood top on and introduce humidity into the box with a humidifier or pans of water. Can't guarantee that the stain will be unaffected or grain raising won't occur but I don't know that you have a choice.

1/12/22       #4: Cupping table top midway through fi ...
RichC

You get distracted on customer work all the time? You must have a great cash flow to just ignore work.

1/12/22       #5: Cupping table top midway through fi ...
RichC

Sorry, hit enter too soon. Nothing I know except ignore it until the humidity comes back up this spring. Anything else you try to do will affect the stained top, or you have to rip it down and start over. You don't have any cleats glued and screwed on the bottom do you? Or have the top hard fastened to the apron? A 46" wide top will shrink about 3/8" in width when the furnaces start running.

1/12/22       #6: Cupping table top midway through fi ...
Mark Ragnar Bakunda  Member

Website: http://www.ragnarfurniture.com

Tim, If you are running the heater in the day, I would put it in your veneer vacuum bag with a moist rag in a separate part of the bag but wrapped in the breather mesh connected to the top. I would leave that overnight.

I normally find that it is less stress and a better quality product I put out when I don't patch repair, but strip/scrape/sand to bear wood and start fresh again.

1/12/22       #7: Cupping table top midway through fi ...
RichC

What happens to the stained top with a damp rag on it overnight? Can't be good.

1/12/22       #8: Cupping table top midway through fi ...
Matt Calnen

Going on the premise that it will come back eventually, would it be possible to force it flat right now by adding c channel to the bottom? I have a friend who has used these quite a bit to hold them flat. After all, a wide slab will be prone to cupping with the plainsawn grain in the middle and the radial grain at the edges.

Something like this- https://www.etsy.com/listing/971648604/34-c-channel?gbraid=0AAAAADtcfRIF-BuiviH00i
AoZNF2hSG-S&gpla=1&gao=1&&utm_source=google&utm_medium=cpc&am
p;utm_campaign=shopping_us_a-craft_supplies_and_tools-tools_and_equipment-home_im
provement_and_hardware-other&utm_custom1=_k_EAIaIQobChMIp83n6tet9QIV7MmUCR2QI
A-VEAQYCCABEgKrXfD_BwE_k_&utm_content=go_12573075997_120353203420_50785173924
7_pla-295462057867_m__971648604_118659680&utm_custom2=12573075997&gbraid=
0AAAAADtcfRIF-BuiviH00iAoZNF2hSG-S&gclid=EAIaIQobChMIp83n6tet9QIV7MmUCR2QIA-V
EAQYCCABEgKrXfD_BwE

1/12/22       #9: Cupping table top midway through fi ...
Matt Calnen

The link didn’t work, how about a picture instead.


View higher quality, full size image (750 X 1334)

1/13/22       #10: Cupping table top midway through fi ...
Tim Member

I should be using C channel.....I have not. While surely will help with future movement; I am skeptical it would work to "force" it back to flat

1/13/22       #11: Cupping table top midway through fi ...
RichC

No one "should" be using steel under a wood table. With properly dried wood and worked with the understanding of how wood moves, it's totally unnecessary. How many centuries of table making do you think have gone on with no steel under a tabletop? I've been working wood for 50 years this fall. Not once have I put steel under a tabletop.

1/13/22       #12: Cupping table top midway through fi ...
the google

i've done the c-channel (uni-strut) thing before to try to "reflatten" a table top. it didn't work for me. your experience may be different, but i think it will be a fruitless adventure. the guys doing this, and for reasons besides keeping it flat, are doing it when the piece is already flat.

1/13/22       #13: Cupping table top midway through fi ...
Matt Calnen

Rich C, why is it no one “should” use a piece of steel to hold a top flat? If a design or style doesn’t call for the use of a breadboard end, or apron below, what is wrong with it? It does the job. While the design and style are not of my liking, someone must like them as you see a lot of them being made these days.

With the radial grain at the center of a wide slab, it will always want to have some face deflection from flat during seasonal changes in humidity, properly dried or not.

1/13/22       #14: Cupping table top midway through fi ...
RichC

To me it's about craftsmanship and study of your medium of your choice. I know that flat sawn from juvenile trees (or logs sawn through and through) will always move or cup, so I don't use it. I never use flat sawn wood for a wide tabletop so I have that advantage. I don't ever have to fix up something that will be known to cup or move. Slapping steel on the bottom has been done wrong so many times, and it causes more trouble when slots are not used for the screws. That C channel system has very short vertical legs. That is where the bending strength is. I also know that uneven finish or lack of air moment around panels is also a disaster. It's been written about very often and I trust the knowledge of the others that came before me.

1/13/22       #15: Cupping table top midway through fi ...
Matt Calnen

Well Rich, I guess you can’t teach an old dog new tricks. To clarify my point, we are not talking about slapping steel on the bottom. The C channel is very rigid, much more so than angle iron. It clearly has elongated holes to allow for movement. On a 10’ long top you could put 4 or 5 of those on and it will help immensely. I have seen many wood island tops made this way. The last one was made of two15’ long, 21” wide Bookmatched Sipo boards. In the modern style of the kitchen, those where the only way the maker could hold them flat. I’d consider that use of the metal “necessary “.

If you are interested in enriching your knowledge base, and learn from history, I’d encourage you to visit museums and view early American furniture built with beautiful wide boards, and no, they didn’t use metal to flatten large tops, but I bet if they had them they would have.. You don’t need to limit yourself to straightgrained wood like you indicated. You should also look at what the SAPFM members are making, using nice wide lumber. Remember why we started working with this material, it’s beautiful!
Food for thought, GM has been making autos for about 100 years, I’d say there are a lot of other makers out there half that age that make a much better car at a better price. Experience is valuable, but we must always be open to learning new things and applying the good ideas where appropriate.

1/13/22       #16: Cupping table top midway through fi ...
RichC

Matt
Are any of the SAPFM guys putting channel iron under their tables? The lumber that came out of old growth 500-1000 year old trees used in early American furniture is nothing like we see today. The high end shops on the east coast were using Cuban Mahogany boards that were 3-4' wide. White pine in Maine could have been that same size if they weren't marked with the King's Broad Arrow for ships masts.
I agree with "the google", that steel channel is not rigid enough to take the cup out of a 46" panel. But honestly we don't know the thickness of that table top. If's 6/4, It definitely has more strength than the steel.
Finally, reinforcement under a table is not a new trick. Consider a tilt top table that has two large cleats under the table for strength and to serve as the hinge members.
I don't have to learn your new trick, I'm very happy with just using wood in my tables. At this stage of my life (I'll be 70 this year), there's a good chance I won't make another table. I'd have trouble turning that 116x46 table top over right now, let alone carry it out of my shop. Flipping stuff like that all my life has certainly taken a toll. Got a real nice report of arthritis in my lower spine just recently.

1/15/22       #17: Cupping table top midway through fi ...
Chemmy  Member

Seeing how there should not have been a movement in moisture content to any appreciable amount unless the humidity was way high for the two weeks noted, it sounds like it is the problems I had with piano lids/ tops in FL. I would spray them with heavy coats of polyester and let dry till the next day and they would cup, but...that was from the shrinkage of the polyester film.!!
When the same amount of polyester was sprayed on the top the next day, it would flatten itself out over night.
Not sure about about leaving so long as you did, but I would at least try and see if this either solves the problem or considerably helps.
If so then at least you know what caused it for sure and if there is any cupping left after 2 Weeks, another coat or so may still level it out completely..!
The only drawback is the time factor, if you now are under the gun to get it done and delivered.!!

3/13/22       #18: Cupping table top midway through fi ...
Fred Melton

Website: http://Meltonllctx.com

There is a very simple fix put the table in a small room with humidifier raise humidity up to 40 to 50% for about 4 to 5 days when it flattens finish it jobs done I have built several hundred tables an put finished on several thousand tops .If it cupped in 3 or 4 days all you have to do is reverse it an problem is solved .It can help in winter lower temperature in room to about 65 in winter


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