Seal bottom side of wood bathroom vanity top or not?

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?
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Seal it. It's a bathroom, high humidity area and eventually, if conditions are right, mold will form. Not a good thing.
Bob S.
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Seal it. If the two sides have uneven moisture content, it will warp. Use the same stain and epoxy coats as the top so both sides are treated equally.
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I'm a countertop specialist. Seal it. Seal the hole for the sink edge as well...and the tap hole(s)
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On Thu, 11 Mar 2010 14:20:07 -0800 (PST), the infamous Robatoy

Has Alan from Oz piped up yet? I haven't seen his post if he has. He's had jarrah countertops for a couple decades and should have some really good input, too. No doubt he'll confirm your advice, Toy.
-- There is no such thing as limits to growth, because there are no limits to the human capacity for intelligence, imagination, and wonder. --Ronald Reagan
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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.

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Good info and I certainly would seal it all around just to make sure but....
It begs the question, why don't you seal white oak flooring and boat decking all around?
Bob S.
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.
snipe of good post.....
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Dam fine question. Not really sure on that one.
My speculation: on flooring the underside is pretty concealed from the environment, not getting any air flow for exchange of moisture in\out. Also, I seem to recall one fo the forst steps in doing floors is to determine the moisture content of the substrat and to apply moisture barrier in most cases. In fact, certain floors are not supported for us over concrete floors IIRC.
Boats, I guess they are going to get pretty saturated from the ambient in a fairly consistent manner and kind of always stay at a high saturation.
... all speculation, but really just slighly more so than most of my posts ;^)

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Well at least in the old days when the subfloor was just loosely spaced and knotty wide planks the underside would be pretty exposed. On the first floor, the undeside would get pretty good circulation with damp basement air...

Though in the Northern climates, Boats get taken out of the water in the winter so I would think the range in extremes from total saturation in the summer to total dryness in the cold winter air in the winter would be a lot more extreme than say a bathroom where the moisture level is high but all-in-all pretty uniform over the time scale that wood exchanges moisture with the environment.
Also, I would think that boats would be particularly susceptible to such warping & stresses since the wood is often "bent" and applied under stress. Plus the fitting needs to be particularly tight relative to countertops and flooring where a little expansion/contraction/warping may be ok.
n> ... all speculation, but really just slighly more so than most of my

All good and helpful speculation, but for me at least the question remains...
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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.
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Since I am using epoxy as my topcoat which is quite impervious to water do you think I really need the SAME number of coats on the bottom side? I was planning on applying 4-5 coats on the top to build up a good thick physical barrier in addition to a moisture barrier. Since the underside only needs to protect against moisture and since epoxy is pretty moisture impervious, can I geta away with maybe just 1-2 coats on the bottom. Also, on the bottom, since I am not looking for a perfect smooth finish, I could apply each coat a bit thicker to compensate.
Does this make sense?
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Yeah, I suppose a good coat on the bottom is all ya need.
I also agree, oil stains can be sketchy under any film finish. I think gel stains are a lot safer because they conatin poly and will dry out for sure, but be careful is best.

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As with most wooden furniture, finish ALL sides to keep it as stable as possible. I'm sure more are left unfinished than not, either due to cost or laziness.
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