Interesting Article

[tvgordon]

I just read an interesting article in the July 08 issue of Woodshop News.  It was written by Bob Flexner and in it he says that the need to finish both sides of a board is a myth.

He states that many older pieces were not finished on the inside and suffered no cupping of the boards.  The earliest reference to the need to finish all board surfaces was in 1976.  He reasons that if the inside of cabinets, etc were finished, it was for aesthetics and not needed to protect the wood from humidity changes.

He found in order to halt moisture into and out of the wood was to apply an 1/8" thick coat of epoxy.  Of course the finishes commonly applied are no where near 1/8" thick.  It is more important to start with stable, dry wood.

He says the board will cup on the heart side (of a plain sawn board) independent of how the wood was finished.

Now, I am in no way a finishing expert, but it seems to me that if only one side is finished, the unfinished side will change in moisture faster than the finished side.  Wouldn't  it be possible that the board cups if only temporarily until the finished side changes (at a slower pace) an equal amount?

I remember on another post where someone asked about his workbench top warping.  Most everyone who responded asked if the top was finished on both sides.  So it seems that most woodworkers feel that it is necessary to finish both sides of the wood.

Something to think about.

Tom.

 

[Warrior]

This is only somewhat related.

I have in the past moistened the cupped side of a plank to flatten it out. Then added gussets to keep it flat.

 

[Michael Kellough]

One of my current projects is to flatten a large slab table that is only finished on the top.
(Slab as in a large board that sits on cabinets rather than having an apron and legs)

It is humid here now and the finished side is cupped. I think I'm going to have to build a plastic tent around it and put a dehumidifier in there to try and flatten it.

Do you think that will help?

Whether it does or not I'm going to seal the bottom with a couple of coats of something and add cauls to keep it flat.

 

[Warrior]

How thick is it?

Can you refinish the top?

 

[Tim Sproul]

I just read an interesting article in the July 08 issue of Woodshop News.  It was written by Bob Flexner and in it he says that the need to finish both sides of a board is a myth.

I have not read the article but this is not the first time Flexner has 'said' this.  It makes sense because almost always, the unfinished side is never exposed to the open enivironment.

Drawers - the sides, back and the inside surface of the front are often unfinished or left with just a smattering of shellac.  The case - the inside parts holding the case together and providing runners and kickers for the drawers are often left unfinished....

Finish trim in a house.  Do you really prefinish all the trim - all 4 sides and the endgrain - before installing?  I've not noticed cupping of baseboards being a very common problem.

Even if you finish every surface of every piece of wood that goes into a project, every surface does not 'see' the same environment/humidity.  It takes a lot longer for a swing in humidity to work its way into the backside of chair rail compared to the exposed side.  Same goes for a door panel on a cabinet.  The inside surface of the panel doesn't see the same humidity as the outside surface (most people do keep doors closed except for the few moments to get or replace items) at the same time.

 

[Warrior]

I think that furniture items and cabinetry are more relavent to this article.

 

[Michael Kellough]

How thick is it?

Can you refinish the top?

The table I'm working on is maple, 44" wide and 3/4" thick with a 1 1/4" molded band all around dovetailed in the ends. The table moves a good 3/4" in width through the seasons and the finish in great condition and must be preserved.

The guy that made this table is one of the best makers of Windsor chairs and also has a lot of experience restoring valuable antiques. He would agree completely with Flexner. However, the wood he used on this table apparently wasn't completely seasoned and since the slab isn't braced by any structure I think the decision to leave the bottom unsealed was a mistake.

I think it's best to seal all surfaces with the same material and at the same time if possible. This of course would add a lot of cost to the piece so best isn't always the right choice.

 

[Warrior]

Michael or Mike?

Since the top is only 3/4 you should be able to get it flat and keep it flat with the cauls only. If you can take the top off, you could clamp it flat with temporary cauls and route the bottom for permanent sliding dovetailed cauls. Forget the tent, dehumidifier and even putting the finish on the bottom side. As long as the sliding dovetails are a good fit the top will stay flat. JMO

Eiji

 

[Warrior]

Depending on how long the top is you could probably be in and out of there in 1/2 a day plus the shop time involved in making the male caul components. Or a bit more if the work is done all on sight.

 

[Michael Kellough]

Michael or Mike?

Since the top is only 3/4 you should be able to get it flat and keep it flat with the cauls only. If you can take the top off, you could clamp it flat with temporary cauls and route the bottom for permanent sliding dovetailed cauls. Forget the tent, dehumidifier and even putting the finish on the bottom side. As long as the sliding dovetails are a good fit the top will stay flat. JMO

Eiji

It will be even easier than you suggest since the thicker band around the perimeter prohibits sliding dovetails. It'll just be screwed on cauls.

I am started to reconsider adding finish to the bottom since that will delay adding the cauls, but, I think the owner is set on adding it.

 

[Eli]

well that's an easy answer then. Who cares if it works if that's what he wants?

 

[monte]

qoute from tvgordon....
I just read an interesting article in the July 08 issue of Woodshop News.  It was written by
Bob Flexner and in it he says that the need to finish both sides of a board is a myth.
quote from michael......
One of my current projects is to flatten a large slab table that is only finished on the top.
(Slab as in a large board that sits on cabinets rather than having an apron and legs)
It is humid here now and the finished side is cupped. I think I'm going to have to build a plastic tent around it and put a dehumidifier in there to try and flatten it.
Do you think that will help?
Whether it does or not I'm going to seal the bottom with a couple of coats of something and add cauls to keep it flat.
The table I'm working on is maple, 44" wide and 3/4" thick with a 1 1/4" molded band all around dovetailed in the ends.
The table moves a good 3/4" in width through the seasons and the finish in great condition and must be preserved.
The guy that made this table is one of the best makers of Windsor chairs and also has a lot of experience restoring valuable antiques. He would agree completely with Flexner. However, the wood he used on this table apparently wasn't completely seasoned and since the slab isn't braced by any structure I think the decision to leave the bottom unsealed was a mistake.
I think it's best to seal all surfaces with the same material and at the same time if possible. This of course would add a lot of cost to the piece so best isn't always the right choice.


Allow me to interject here........

If a piece is secured by a structural frame I could see skipping the step of finishing it all around.
(this would also depend upon how it is secured, but that could be another post)

If it is not secured, such as a slab as Micheal's client or a cabinet door I personally would always finish the unseen side.
If it's not seen, it doesn't mean that it is not absorbing moisture from the surroundings, even on a closed door cabinet.
(Keep in mind that I came into woodworking from being a finisher for years first, and working on a lot of antiques at that)

I would disagree with Flexner if he is using that as a blanket statement (having not read said article)

Micheal, your assumption was correct......
Now with respect to your project, my first course of action would be to sand the bottom surface first - to open the wood cells.
(I don't believe you mentioned how many years, since it being made) My second step would be to apply a couple of coats of a diluted finish of whatever your guess would be of what was applied on top. (waiting between each coat).....to allow it to really absorb within the maple (esp because it is such a dense wood) I would then apply a thicker coat and wait.

Something is going to move.

Flipping it, against the cup with a two by four on edge running across the width, clamped at either end to the
(mft table below?) as the finish is curing (with some wax paper between the 2 by 4 and the finish) could help.

I'm not so sure if adding the cauls without sealing it first, at this stage would be a complete success over time.
(you might have him sign a waiver, if you do go that route)

anyhow, my 2 cents before my morning coffee,
Roger

hmm, any chance to see some photos?

p.s.
qoute by underused.....
I've also put a warped board out on damp grass in the morning, and had success flattening it back.
......that could work on a board that is not finished on the side not hitting the grass, if you do that on a board that already has a finish on the otherside - such as Michael's, I believe you could make it worse. Remember, it was excess moisture from the unsealed side which stared this mess in the first place.

 

[monte]

Michael,

Also, a post thought........
Since the ends of the board are wrapped,
and your end grain is not open, you might consider routing a channel at the bottom of the board along the end
grain - just inside from the wrap......keeping that unsealed - and sealing it only after this process is completed.
(good excuse to run out and get an MFK 700 ;D)(hmmm, check for an occasional nail first :-\)
After sealing the bottom, .......the routed channel should help the moisture escape.

Roger

 

[CharlesWilson]

What Flexner wrote in the article is true during equilibrium conditions (when any absorption, evaporation of moisture from both top and bottom has ceased). Here in New England, there can be wild swings of short duration in relative humidity. For Michaels table top, with its 3/4 inch variation, an unfinished bottom would be growing and shrinking more rapidly during these transition times causing temporary bowing and cupping stresses until an equilibrium is reached.

I don't know what the best compromise is, but there may be some benefit to sealing both sides of a table top.

 

[Jerry Work]

Hi all,

Finish all components in the flat before they are assembled and this question will never even come up.  Since I never know where a piece of my furniture will wind up I would never even consider finishing only one side.  It may live with the original person who commissioned it to begin with, but their heirs may live anywhere.  No reason to take chances spoiling something you spent so much time and energy creating in the first place.  My philosophy anyway.  Cheers.

Jerry