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Varathane over stain…required?

unclejemima | Posted in General Questions on

Ok guys. I’m sorry, this is not really a “green” building question, but there is such knowledgable members on this forum I had to ask…I can’t seem to find a solid answer anywhere! Plus it deals with VOC’s, so thats green based 🙂

I have douglas fir 6×6 beam and 1×6 t&g. 50% will be used indoor, 50% will be used outdoor.

We stained the fir with a oil based stain labeled as “interior and exterior”. I could not find a water based “exterior” based stain and liked the idea of using the same for all to keep the color matching.

The wood is sandblasted so its really rough (to make it look aged).

My question is…do I “need” to use a varathane or similar top coat over the wood? I can see it providing protection for the exterior use (its under a soffit so it won’t see any sun) but for interior is it needed?

Is there any sort of VOC protection by using the varathane over-top of the stain, to sort of “lock in” the VOC’s of the stain? (this is more relevant for the interior areas)

The varathane I’m wanting to use is “Diamond Wood Finish Water Based”

Looking forward to the kind advice 🙂

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  1. GBA Editor
    Martin Holladay | | #1

    You didn't really do a very good job of explaining where these T&G boards are being installed, but the few clues you provide allow us to conclude that you probably have a board ceiling -- I'm guessing that the ceiling boards double as your roof sheathing -- and that the boards installed in this location penetrate your home's air barrier and extend from the interior to the exterior. On the exterior, these boards are a type of soffit. Is that correct?

    If so, polyurethane decisions are the least of your problems.

    The main problem with this situation is air leakage. It's extremely difficult to stop air leaks that occur where the boards penetrate the air barrier.

    To address your question: Polyurethane is optional in both locations (interior and exterior). If you aren't sure what your boards will look like, with and without polyurethane, play with some samples to figure out what you like.

    Your exterior boards will change color with age. If you decide to install polyurethane on the exterior boards, the polyurethane will delay the aging process, but not stop it.

  2. charlie_sullivan | | #2

    And on the VOC issue, many of the VOCs you might be concerned about are smelly, so your experience of the smell is an OK guide to when the VOCs are coming out. Just a guess, but it's probably mostly in the first day, a little in the first week, a tiny bit over the first month, and very little after that. If you coat it with polyurethane, you will stretch that VOC emission out over a longer time. The total amount of VOC released will be the same, but it will extend over a longer time. Is that better? Maybe, as you are never exposed to high levels. On the other hand, if it smells bad for a week, you are motivated to open a window whereas if there is a smaller amount over a longer time period you might not bother, or it might be bad weather for opening a window. So your total exposure to VOCs might be more that way.

  3. Expert Member

    I take your point about letting VOCs off-gass for an initial period, but locking in VOC emitting finishes does significantly reduce the exposure of building occupants. If you take them below certain threshold levels they are diluted to a point that, as far as we know, they have no effect on our health. Long term exposure to next to no VOCs is still preferable to short term high levels.

  4. unclejemima | | #4

    Thanks for the answers guys.

    To clarify, the T&G boards will be used under a non-vented soffit (spray foam to roof deck) so its purely cosmetic.

    Regarding the VOC off-gassing where the T&G boards and the 6x6 beams are being used for interior, the use of a polyurethane will reduce the VOC level to something that will slowly dispate over time...

    Regarding interior use of the T&G (as cosmetic ceiling in our kitchen) I'm wondering if I should then should leave the wood without polyurethane until closer to moving in, the oil stain would have dissipated more... then final application of polyurethane will now mask the remainder to a much lower level safe for indoor occupants.

    Thanks for the advice,

  5. unclejemima | | #5

    Can anyone comment on if I should varathane both sides of the T&G for use outside, if only the one side will be visible? Does it protect the wood any more to do both sides?


  6. GBA Editor
    Martin Holladay | | #6

    For a soffit, I don't think you need to polyurethane both sides of the boards. You can if you want, though.

    These boards won't get any direct exposure to rain.

  7. unclejemima | | #7

    Thanks Martin :-)

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