"SonomaProducts.com" writes:
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.

OMG - no apologies - I'm the one who should be THANKING you for that
incredible lesson.

Regarding the grain, I ended up filling it with thinned down Wundefil
Wood Filler (which is recommended for that usage and surprisingly is the
only product that Rockler had for filling grain other than some
water-based clear fill). I used walnut which seemed to be the darkest
version they had in stock.

I typically (in my own amateur way) like to play around with mixing
stains and gel stains -- and have had good experience with General gel
stains. HOWEVER, in speaking with West System tech support, they
referred me to an article on "Epoxy adhesion over
stains". http://www.epoxyworks.com/21/epoxy_adhesion.html

Suprisingly (to me), the popular minwas oil stains FAILED the adhesion
test even after 4 days of drying. This (and a follow-up phone call to
tech support) has led me to be very cautious about which brand of stain
to use. Unfortunately, the list of tested stains is rather short and
doesn't include the General line. From personal experience, I have found
the General gel stains to be a bit "tacky" so I was concerned that they
too might fail the adhesion test. The West tech support guy said that
they have long used the Pratt & Lambert Tonetic line so I went off and
bought some.

Bottom line is that based on this, I am being (overly?) cautious about
what types of coating to apply under the epoxy topcoat. For the same
reason, I am staying away from shellac layers etc.

But, I plan to take note of your advice and use it in other projects
where I don't plan on clear coating with epoxy.