Consequences (if any) of sealing only one surface on wood project (black walnut)
Hello all, and thanks in advance for any replies!
I posted a question last night in the general forum about a repair I may or may not attempt, on a clock door comprised of 3/4″ thick walnut framing a somewhat heavy beveled glass panel. My woodworking experience and knowledge is limited to the simplest projects, like having assembled & finished a couple oak and walnut clock kits, and a few very low-level amateur, do-it-yourself projects. I inquired whether the door falling apart could have resulted from moisture issues related to only the visible, outside surfaces of the door having been sealed (polyurethane, I’m guessing, but only guessing; it has a mild gloss to it). I read in a couple places you have to seal all the surfaces or none at all, to avoid problems. The reply (very kind, deeply appreciated) on the general forum said it was a myth that all surfaces (or none at all) must be sealed. Is this in fact the consensus (which would be a relief to me), or could it be more complicated (maybe differing by species of wood – mine being black walnut)? The item is about 30-40 years old.
Thanks for any responses (and apologies to the original responder for seeking confirmation; it’s only because there appear to be a variety of opinions on this; I hope you’re correct & it is indeed a myth!!).
Replies
I will chime in with the understanding that make no claim to being an expert on the subject. And, I have no scientific proof one way or the other. I just try to be logical about it.
Some say that it is a myth based, I think, on the fact that they see lots of antiques without finishes on the underside and inside. Maybe true but, I ask, how about other factors like construction methods, species of lumber, how it is sawn, and climate factors that the piece has endured. Further, what about all of the antiques, similarly finished, that have not survived; maybe due to poor finishing techniques and maybe not. You can't judge because they are here to judge. So, statistically speaking, you have an incomplete sampling on which to base your judgement.
I know for a fact that if you lay an unfinished board out on a grassy lawn, depending on the grain, it will usually cup because the bottom side gets damp and the sunny side stays dry. I removed the back panel from an old bookcase a short time ago. The panel was made up of 1/4"x 4" t&g slats. They were finished on the inside but not the back side. They were all cupped. Would they have cupped if finished on both sides? I don't know but, it seems logical that finish on both sides would have, at least, reduced it if not eliminated it.
I think it comes down to "why not"? It seems logical to put finish on both sides and I certainly can't see any harm in it. And, I can see some evidence to support the idea. Of course, doing so would not alleviate the need for proper construction throughout.
Thanks for the response! Yes it makes sense - intuitively, or logically - that there will be some effects, probably adverse, to sealing one side but not the other, but as you say - in agreement with the gentleman responding to my other post - environmental factors will play a significant role. Both of you seem to agree that if there are any such warping effects, they would be more or less pronounced according to fluctuations or extremes of humidity. In my case, it's a clock that has been in a moderate Midwestern climate, without extremes (or extreme fluctuations) in humidity.
Thanks again for the response; which while very much appreciated still leaves the question a bit unsettled; the original reply seemed to think there is no affect whatsoever on the the wood of sealing vs. not sealing it. It was very optimistic in terms of my situation (I got a walnut clock from somebody with this possibly an issue), so my "optimism-pessimism pendulum" swings back & forth.
Thanks, anybody, for **any** feedback on this (in either direction)!
Looks like you are wanting an answer that fits your diagnostic of the cracked wood due to improper finishing, if you keep asking the question on forums you might eventually find it, but how would you know if it’s right?
Other than for wide panels such as table tops finishing both sides is not common practice and not necessary. Wide panels are subject to cupping if moisture is unevenly exchanged from one side of the board to the next so finishing both sides is preferable, I coat the underside of wide boards with shellac for that reason.
In your case the clock is by no means an antique, I have walnut lumber in my rough wood pile that is older than that. A hardwood door frame will not crack or decay because it’s not coated on all sides.
I checked with an old wife and she said put one coat on the back side to make it easier to dust. Her eyes glazed over when I asked her about wood movement, and a quick refresher on moisture content put her to sleep.
Finishing both sides of an object was never done unless both sides were visible. No books or writing ever mentioned finishing both sides to stabilize a piece until 1976, and the myth spread from there.
Floors are not finished on the bottom. Wainscoting and real paneling are not finished on the back. If installed correctly, all those things have lasted hundreds of years, intact and pristine.
Boards left outside, re deck boards, always cup on the top. It doesnt matter if they are oriented heart up, heart down, finished one side or both. That's because of a phenomenon called compression shrinkage. When boards get repeatedly wet on one side only, the cells only on that side swell, then shrink when dry. Over and over. The cells end up smaller than on the unwetted side, and the result is cupping. Finishing the underside will not help, at all
How exactly did it fail? At the joints or splits in the wood. I'd finish both sides just because I'd likely slop some finish over or have drips that go to the other side anyway. I agree with what John_C2 says.
You folks are all fantastic; thanks so much for taking the time to respond in such detail, so thoughtfully! John_C2, in particular thanks for the follow-up after I inquired if your view is the consensus. I always seek confirmation of good news when it's tempting to accept it too readily. I now feel more confident addressing the repair as a pure mechanical-structural issue, so glad I don't have to worry about the wood having deteriorated!
Doesn't mean I'm out of the woods yet on the re-attachment challenges, but big relief that wood strength shouldn't be an issue.
Thank you so much again, all!
_MJ_ -- What does your new wife say?
-- Mac Campbell
This is one of those circular discussions that surfaces now and again. As with most of what we do, it depends. For large solid wood panels I will seal the insides with a 1 pound cut of shellac because it makes me feel better ;-)
For raised panel doors, both sides are finished because . . . we see both sides. For frame and panel side or backs to large carcasses, I do not finish the inside.
What works for me may not work for you. Some of what I do, I do just because it has always worked for me. With very few exceptions my work lives west of the Mississippi and primarily in the south west basin. Pretty mild humidity swings. If I built pieces for Alabama, Massachusetts or the Florida pan handle, they would reflect that environment.
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