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I am building a very large (110" X 48") dining table. I intend to finish the wood by applying Danish oil and a water-based, oil-modified, polyurethane.

Since it won't be exposed to wear, is it necessary to seal the underside of the table?

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  • 1
    According to Flexner the conventional wisdom that you have to finish both sides equally to prevent problems isn't correct, and the state of numerous old tables and other pieces of furniture seems to support this (the vast majority of which were not finished equally on both sides). On your proposed finishing routine, I would recommend you not use "Danish oil" before waterbased poly. If you need superior waterproofing to what the DO can provide then start and finish with oil-based polyurethane, it's a simpler, easier and more reliable finishing method.
    – Graphus
    Oct 24, 2016 at 8:22
  • @Graphus Hmmm...I think you just prompted another question. woodworking.stackexchange.com/q/4885/192 Oct 24, 2016 at 21:23
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    I see much antique furniture, I have not seen old tables with top finished on both sides. Sometimes edge or lip is finished, but not complete underside. Same on tops of desks, fronts of drawers also the same.. Chests and tool boxes another example, always bare inside hinged lid, top, front and 2 ends usually remain flat!
    – Volfram K
    Dec 24, 2021 at 7:07
  • Jeromy, there should normally be other factors that have far more impact on this sort of thing.... hence Flexner's advice, and the similar observations others have made (which I have continued in the itnervening six years, seeing nothing to cause me to alter my position on this). Which way the tabletop warped is critical to a diagnosis; there's a change neither the finish on the top nor the unfinished underside (depending on which way you want to think about it) contributed anything of note! I hope it is evident that how a tabletop is fastened to the aprons is no. 1 of the other influences.
    – Graphus
    Dec 26, 2021 at 14:08
  • P.S. I look forward to seeing photos of your table. Instead of just showing the bow please take photos of any relevant construction details, and the end grain.
    – Graphus
    Apr 15, 2022 at 20:20

5 Answers 5

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American Institute of Architects (AIA) sets the standard for commercial woodworking. The standard requires that the finish be balanced whatever is done to one side must be done to the remaining 3. This prevents the wood from moving in unequal ways.

Essentially you set up a situation where the grain will behave differently on opposites sides of the wood as the ambient conditions will effect the wood differently on differently finished wood. Now the average dining room table may not see conditions extreme enough to cause a problem but that is standard.

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  • I'm changing the accepted answer to Rawhrit's for two reasons: 1) because of the reference to the professional organization and their policy, and 2) because after 6 years, my table (finished on one side) has a noticeable curl during the high humidity summer month. Dec 23, 2021 at 22:21
  • what is the website for the AIA? aia.org? Dec 23, 2021 at 22:22
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    Link please! And, remaining 3? 6 sides to each piece of wood,
    – Volfram K
    Dec 24, 2021 at 7:03
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    @JeromyFrench, please add photos of table with 'curl' to question!
    – Volfram K
    Dec 24, 2021 at 7:05
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    @JeromyFrench better yet, post a whole new question about why your table curled.
    – FreeMan
    Dec 24, 2021 at 15:27
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Bob Flexner says it is unnecessary to finish both sides of a table top:

...finishing the undersides of tabletops or the insides of cabinets or chests has only limited impact on reducing the likelihood of future problems. The only reasons to go to the trouble are for looks and feel — both of which are perfectly legitimate. But neither has anything to do with stabilizing the wood...

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No, sealing the underside of the table is not necessary.

Wood species, dimensions, grain patterns, moisture levels, and how the boards are joined will be the primary factors regarding movement.

The only reason to seal the underside is for the aesthetic appeal of a fully finished piece.

My current dining table is walnut veneer over walnut with an unfinished underside. It was made in the 1950's and is still in excellent condition.

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I think it should be considered depending on the use the table is intended for. I deal poker in a casino where I sit at the table as high as I can, this means my legs are often up against the bottom of the table. I have picked up a lot of small splinter over the years. I think in some cases it would be nice if there had been some finish on there that would of kept the grain and splinter from rising.

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IMHO, any tabletop or large panel surface should be sealed similarly on top and bottom.

A lot of people throw together panels, brush some stain and a finish on them and that piece of work is now seriously susceptible to a lot of atmospheric changes on only one side of that large piece of human interacted lumber.

I've seen splitting and warping due to this and simply recommend at least 1/4 to 1/2 of the coats of finish be performed on the bottom compared to the top surface.

Minimal 'extra' work and consumables to produce a finer longer lasting piece.

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