Like DJ said, seal it all over with the same number of coats to avoid
warpage. The issue is the ambient moisture much more than the spills.
It takes weeks for the moisture to enetr the inner cells of a board so
the humidity in the bathroom is the real issue. If it is moisting and
drying from one side only you will introduce stress, of expansion and
contraction thus warpage. Try laying a newly glued up panel flat on a
concrete floor and in a day or two you'll see what I mean.

White Oak won't be rotting on you any time soon. Even if there is
intrusion I wouldn't worry about it. They are still digging up old
wood boats of English White Oak and that wood is still in good shape,
a few splashes of tooth paste isn't going to hurt anything.

Also, if you have the option and like the look, go with quartersawn
and it will be even more stable and resist warpage.

Not sure about what look you are after but all Oak (I think) can
really benefit from some grain line darkening. You can do it without
all the trouble of classic grain filling which is really about
flattening any way. I stain or dye to whatever color I am after, then
do a thin wash coat of shellac, maybe a one pound cut. Then use a
super dark gel stain (I use General jet black) and rub it into the
grain then wipe it all off across the grain and all the grain lines
will now really pop. This is super dramatic with red oak but good on
white also. Gel stains have poly inthem so you can coat directly over
that with anything, or nothing. Not sure if epoxy solvents will free
the gel stain so maybe another barrier coat of thinned shellac after a
week of the oil stain drying, then epoxy.

Sorry for unsolicited finishing advice but I am doing finish
formulations and testing lately so my head is all around this stuff
right now.

On Mar 10, 8:35*pm, blueman wrote:
As per previous posts, my wife insisted on replacing our old rotten wood
vanity top with a new one which I am making out of stained white oak
with multiple clear coats of West Epoxy on top and on the edges...

The question is whether I should seal the bottom (whether with epoxy or
polyurethane). However, I wonder whether sealing the bottom would be
more of a bad thing since if water does penetrate the top then it has
nowhere to breathe out of and since the bottom side doesn't/shouldn't
get wet maybe there is no advantage to sealing it.

Any recommendations?