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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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.
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...
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.
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.
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.