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Discussion Forum

Finish for wood ceiling at bathroom

buildboy | Posted in General Discussion on January 19, 2004 11:37am

I have a T&G pine ceiling in a bathroom that needs a clear finish of some sort. I don’t like the thought of shiny poly. Any thoughts out there for other options (consider the moisture)? Thanks in advance.

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  1. Scooter1 | Jan 19, 2004 11:45pm | #1

    Exterior Satin Minwax Varnish. Apply it in thin coats, wipe it down, and if it is still too shiny, knock down the finish with some 600 grit wet dry sandpaper or some 0000 steel wool.

    Regards,

    Boris

    "Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1934

  2. User avater
    goldhiller | Jan 20, 2004 06:46am | #2

    I fear you may have some problems looming on the horizon already if the wood is mounted to the ceiling and there's no finish on the back side. For the theoretical best resistance to cupping tendencies, moisture absorption and dissipation should be uniform on both sides of a board. Water vapor is likely going to migrate thru the t&g and then be readily absorbed by the backside if totally unfinished, threatening to cause excessive expansion and cupping. If this is the case and there's a vapor barrier in place under the boards (which means the vapor has nowhere else to dissipate).... I'd make certain that an adequately sized vent unit is in place and that it's used nearly to excess.

    Shellac is a very good vapor barrier/retarder. Several coats would offer quite a bit of resistance, but it too will be quite shiny. I'd follow with a couple coats of oil-based poly to add to the vapor resistance and to supply a surface receptive to cleaning. A satin poly doesn't have much sheen at all compared to many other available choices. For the shellac, I'd recommend a dewaxed product like Zinnzer's Sealcoat to insure good adhesion of the poly.

    (A wood ceiling in a bathroom is an iffy proposition for the most part)

    Knowledge is power, but only if applied in a timely fashion.



    Edited 1/19/2004 10:48:17 PM ET by GOLDHILLER

    1. Flyingvranch | Jan 20, 2004 04:34pm | #4

      Hmmm... I own a house that has had that same type of ceiling for 65 years (including bathroom) and none of those terrible things that you mentioned have happened yet.... Who would actually get up in the attic and finish the topside of their ceiling ?

      1. Mooney | Jan 20, 2004 05:43pm | #5

        Gold Hiller has laid some exellent advice even though it hasnt happened to you . Which brings me to the thought of how yours was done ?

        Whats hes talking about is four side priming with shellac "before installation".

        Four side priming is done close to salt waters that has a constant moisture content . Hes just using common sense arrived from that to a bathroom ceiling done in wood . As he mentioned its still iffy because of future discoloration . Four side priming would be the very best method if the wood has to go up. If its already up then its really risky.

        It would also depend on how the user treated the bath by not taking steaming showers  , but who would know ?  Ive seen a lot of baths with vent fans not working and i suppose there are baths with out the fans installed at all . That used to be the practice if a bath had a window . Trouble is no one wanted to raise the window to take a shower . LOL.

        Tim Mooney

        Tim Mooney

      2. User avater
        goldhiller | Jan 20, 2004 06:15pm | #6

        The boards would be finished on the backside prior to mounting, not after the fact. If you can get at yours, then it may mean that the migrating vapor has somewhere to escape to rather than being presented to and absorbed solely by the naked wood on the back of the boards and that this has been a saving grace.

        The same principles hold true for say a tabletop comprised of solid wood. It needs to have the same amount of finish on the top as the bottom or cupping results.

        A look at some other examples of wood and commonly accepted finishing practices tells us that most solid wood flooring doesn't get finish on the underside. Some cupping usually results if laid on the first floor over a damp basement or crawlspace for example, while very little if laid over a dry basement. Other factors come into play as regards flooring because the bottom side isn't exposed to the same air exchanges as the topside. AC or the lack of it can play a hearty role in the control of cupping as well in these circumstances.

        A steamy bathroom environment coupled with a wood ceiling having no finish on the backside of the wood ceiling boards raises the odds that problems will result. However, if one lives in an extremely dry environment, then the water vapor produced from showering for example would likely be so readily absorbed by the air in the bathroom that the odds of cupping ceiling boards would be notably lowered. This is one reason why I said that wood ceilings in bathrooms are an iffy proposition.

        Ideally, to prevent cupping of boards......you're shooting for an even rate of absorption and dissipation on the front and backsides of the boards. This can be a difficult thing to accomplish when the exposure rate is different on the two sides. The back of a board may only be exposed to 10% of what the front side is. One would then theorectically want 10% as much finish resistance on the backside as on the front. Since it's impractical to run a study of the differing exchange rates of the two sides, etc. on every project........the best practical approach is to apply finish in a manner that brings the rate of exchange as near to zero on both sides of the wood as is practical.

        Other factors that play a role in success or failure of avoiding cup or buckling due to expansion on a broad expanse of wood are the width of each individual piece, the species and whether that stock is plain or quarter-sawn.

        What works in one situation won't necessarily work in another. I find the best approach is the one that works the best in a wide variety of circumstances. That way I'm less apt to find myself saying " Damn, I shoulda" in the future.

        Knowledge is power, but only if applied in a timely fashion.

        1. Flyingvranch | Jan 21, 2004 05:26pm | #13

          I do want to apologize for the way that my post came across as it was not intended to sound aggresive. (Trying to quit snuff!) Anyway my ceiling is unfinished on the topside for the entire house and on the bottom side of the pine floors also. I live in a very humid part of Texas near near the coast. I don't have any cupping problems though. My bathroom is tight and small and I am aware that steam is coming up through the t&g seams in the ceiling. I just feel that with good ventilation that this is a minor problem and so far I haven't noticed any ill effects. Now if the bathroom is a continual sauna.....yes I would do something about it. At the moment I am ok with the way that it is. Thanks for all the good replys. (interesting topic).

          Bud

          1. User avater
            goldhiller | Jan 21, 2004 05:52pm | #14

            No problemo. Didn't take it as agressive anyway, just as questioning.

            Getting wood to "behave" is sometimes easy, sometimes not. I know best how to deal with it in our upper midwest environment, but when I used to do the national show circuit and sold thru galleries from one end of the country to the other, I had to insure that pieces "behaved" no matter where their new homes would be. Getting a call from someone in Denver who purchased a piece that was built in the humidity of our environs and had now done "the nasties" wasn't any fun. You either catch on quick and respond accordingly from the get-go on future pieces or face more undesirable outcomes that cost money to make right.

            There's all manner of considerations that must be taken into account when making these engineering and finishing decisions concerning wood, but if there's one universal solution to preventing cupping, no matter the species or or cut of the wood, it would be to insure that the exchange rate/exposure to atmospheric changes is even on both sides. If this is possible, most any choice of finish regardless of its resistance to vapor exchange will work nicely in that regard. If this isn't really practical on any given piece/set of circumstances, the next best thing would be to head for an exchange rate as near to zero on both sides as possible and so that's what I frequently advise if giving brief coaching/suggestions. The important thing is for someone to understand what factors are at play and then they are better armed to think it thru and make decisions.

            I hate saying "Damn, I shoulda". :-) Then again, it leads to lessons that aren't soon forgotten. Unfortunately, it seems that 90% of learning is the result of a negative experience. Works though. I don't forget when that happens.

            Knowledge is power, but only if applied in a timely fashion.

      3. archyII | Jan 21, 2004 03:06am | #10

        65 years ago they had decent tight grained wood.  The stuff you get today moves a lot with the normal change in humidity.  In a bathroom the problem will be greater.  All surfaces should be sealed (including the edges).  Spar varnish is not a bad choice since it remains flexible.

    2. buildboy | Jan 20, 2004 11:43pm | #8

      Thanks for the advise. To clarify, however, the ceiling is the roof roof sheathing. Boards are 2x6 T&G with rigid insulation above followed by torch-down. There is no attic in this area. Currently the area was simply stained. However, what was once a bedroom is soon to become a bathroom so I felt some sealing of the wood is needed, though the wife and I like the "natural" look. Thanks again.

      1. User avater
        Dinosaur | Jan 21, 2004 06:41am | #12

        Don't worry about sealing it; that roof deck is more than likely red pine, a naturally very 'pitchy' wood. If you stain it with a transparent stain like Minwax, and then oil it with Teak Oil (NOT 'Tung oil'), it'll be fine; the wood will breathe and the water and moisture won't hurt it a bit. It's certainly not going to cup any more than it already has.

        Teak oil needs to be put on in stages. The old rule for oiled-finish furniture was: Once a Day, for a Week. Then Once a Week for a Month. Then Once a Month for a Year. Then Once a Year for Life....

        For your ceiling, I'd say wipe three or four coats on over a week's time, then hit it again once yearly when you do your spring cleaning.Dinosaur

        'Y-a-tu de la justice dans ce maudit monde?

  3. BillMcG | Jan 20, 2004 07:57am | #3

    I'd be tempted to head over to the marine supply house and see what they have on the finish shelf. Spar Varnish comes to mind, but I'm not sure if it comes in satin.

  4. ian | Jan 20, 2004 06:24pm | #7

    don't know if bathroom mould is an issue in your area.  If so you may want to add a mould inhibitor to the finish.

  5. MGMaxwell | Jan 21, 2004 01:26am | #9

    Give me a couple of months and I'll let you know exactly what will happen. I put T&G 1x6 white pine on my bedroom ceiling and master bathroom. Above (behind) that is 6 mil poly that is supposed to keep the water vapor from getting into the cold rafter spaces and condensing. I'm in western NC which is listed as a mixed heating and cooling climate, but at my elevation has little cooling requirement. I thought of sealing all 4 sides but could not bring myself to face the hassle of finding the space and time. This time of year I would have a hard time getting my space to 45 degrees during the day.

    So, now it's up and it looks great, and will look better when I put the poly on and it gets a nice amber tone. I hope I don't get a closer look if it cups and falls off on my head. My salvation (I hope) is my big Fantech attic fan to whisk away all the moisture so that it doesn't end up behind my T&G.

  6. NCLaquer | Jan 21, 2004 04:36am | #11

    Interesting finish challenge.  Someone correctly mentioned that sealing one side while leaving the other open to absorb moisture will likely result in failure.  I don't expect there would be any reliable (atractive) way to seal the t&g to avoid the issue.  If you are intent on a film finish, however, spar is the way to go.  This sold as exterior poly or marine finish.  It dries to a pliable film which will flex with the wood's inevitable movement.  I'd recommend, however, avoiding a film finish and going natural or just oil. 

    The oil (boiled linseed) will not seal the pours of the wood and allow it to absorb the moisture a bit more uniformly on all sides, and avoid the cupping.  So long as there is sufficient expansion opportunity, the wood will regularly expand as it gets wet and contract as it drys.  So long as it has a chance to dry, as I expect it will, it will not cause mold or rot problems.  Again, like someone pointed out above, sealing it would great more of an issue because the moisture will inevitably migrate above, then it would not dry very easily.

    Let us know.

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