Table top moisture

[petertay15]

It is understood that both sides of a table top need to be finished to avoid warping do to humidity.
I have just built a farm table of construction grade lumber. The top has been treated with pre-conditioner and will soon get several coats of Wipe-On Poly.
My question is how similar must the underside be treated? Can I just brush on a coat of polyurethane, or does the bottom need to be treated the same as the top.
Thanks, —Peter

[Willyou]

You will probably get varying opinions. In my judgement, since you have used construction lumber which is not as dry, is a soft wood, and will be subject to greater seasonal movement, you should put as many coats of the same material on the bottom as the top. Even with that, depending on how it is constructed, you may expect some warping and maybe splitting.

[cpolubin]

~ It is understood that both sides of a table top need to be finished to avoid warping do to humidity. ~

According to Bob Flexner finishing both sides isn't necessary.
https://www.popularwoodworking.com/artic...necessary/

Cliff

[petertay15]

Interesting article. Thanks.

[jteneyck]

Flexnor's argument is pretty lame IMO.  Wiping a varnished table top does not result in a moisture gain to the wood underneath.  Film finishes are impervious to liquids, they only let water vapor through.  If you only finish the top it will respond at a slower rate than the unfinished bottom, whether the RH is increasing or decreasing.  You can very quickly see this same effect if you lay an unfinished wood panel on your bench top.  In as little as a few hours it will bow up or down if the RH falls or rises, respectively.  The side lying on the bench is like the top of a finished table; it responds more slowly than the upper (unfinished) side.  

So do you have to finish both the top and bottom the same.  Probably not, but it would be best to put at least a couple of coats on it.  

John

[FS7]

Flexnor's argument is pretty lame IMO.  Wiping a varnished table top does not result in a moisture gain to the wood underneath.  Film finishes are impervious to liquids, they only let water vapor through.  If you only finish the top it will respond at a slower rate than the unfinished bottom, whether the RH is increasing or decreasing.  You can very quickly see this same effect if you lay an unfinished wood panel on your bench top.  In as little as a few hours it will bow up or down if the RH falls or rises, respectively.  The side lying on the bench is like the top of a finished table; it responds more slowly than the upper (unfinished) side.  

So do you have to finish both the top and bottom the same.  Probably not, but it would be best to put at least a couple of coats on it.  

John

Along these lines, I think the following two facts necessitate finishing both sides equally.

1) If you put a board outside in the sun, it will quickly warp predictably based on moisture exchange. Both sides are "finished" equally in this scenario, yet moisture loss differs significantly.

2) If you put a cutting board down on a wet counter, with or without feet, it will warp predictably based on moisture exchange. Again, both sides are "finished" equally, and again moisture loss differs significantly.

It should follow that if you finish one side and not the other, moisture loss will be altered. To what extent will depend on what the source of moisture is (water on the surface, steam underneath, ambient humidity, and so on). For something like a table top, assuming that it is not left wet (spills are cleaned up) then movement should be minimized if you finish both sides equally.

[WoodworkerTom]

Spray the underside with a shellac sealer coat x 2. One or two rattle cans should be enough depending on how big the table is.

Or seal the entire table with shellac and wipe on poly the top for durability.

[rwe2156]

I can tell you one thing, finishing both sides will not prevent cupping.  And finishing one side only does not mean it cupped because of it.

I've built several tables and other furniture with panels over the years, most not finished on the non-show side b/c that's what I learned.  Never had an issue.  Most antique furniture is only finished on one side.

That said, how much more effort does it take? so if you believe in it, do it, but like many issues surrounding wood movement, there's theory, then there's the real world. 

To the OP, regardless of how you do it, with that type lumber there is a good probability of issues b/c the lumber is not totally dry.  Applying a finish will not prevent whatever is going to happen.

[Cooler]

I seal with shellac (I used to use brushing lacquer) mainly because it dries so fast (both finishes).  The shellac's odor is less offensive so I used to seal with that. 

I recently started finishing drawer interiors with Varathane's triple-thick water-based poly.  One coat seems to do the trick, it dries quickly and does not off-gas very much (or at least it does not smell like it does--a factor for drawers as the closed space makes odors last a long time.