Flexner says that tables often cup b/c they are often wiped with damp cloths on the top but, of course, not underneath.
A poster here above said, "No finish will stop the movement of wood. Different finishes slow the movement down a bit. The more like plastic film the more the movement is slowed."
This is true, in part, but the finish is a stronger protection than implied. Again, Flexner: (p. 4) "...a finish in good shape stabilizes the moisture content of wood [...] Inhibiting moisture-vapor exchange effectively minimizes the stresses in wood that can be caused by wide swings in humidity."
Epoxy resin may be an exception to the rule that no finish provides an almost complete barrier against humidity. (p. 168) "...very thick layers can be applied (up to 1/16 inch each), achieving an exceptionally effective barrier against moisture-vapor exchange." (my emphasis)
I've also seen polyurethane applied in extremely thick coats, but it must be difficult to get it smooth b/c the job I saw was pretty messy.
In any event, it is important to maintain the finish after it's applied, in case it starts to thin or wear out.
"Man's most valuable trait is a judicious sense of what not to believe." --attributed to Euripides
Last edited by dglevy; 08-09-2012 at 11:44 PM.