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Community and Q&A

Back priming freshly milled wood for rain-screen siding

maine_tyler | Posted in Green Building Techniques on

Anyone here have experience working with ‘green’ (freshly milled and saturated) lumber and applying ‘finish’ to it? Doesn’t need to stay looking nice as it’d be backside of siding.

To be more specific: I am planning to install some fresh hemlock as board and batten siding. WRB needs to get covered and this wood is the best option right now. There will be a rain-screen gap of 1.5″.

My hope was to get some sort of ‘back prime’ but i’m not sure how possible (or helpful even?) that is with green lumber. The front would get the same treatment for hanging over winter. Warm weather next year would allow for a stain in front. 

One possible formula for wet wood application I found was 50/50 BLO and Turp.

Zone 6A Maine.

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  1. walta100 | | #1

    You do understand that this wood will shrink 10 to 15% in width and thickness as it dries.

    Also the wood is unlikely to remain flat as it dries it is likely to cup twist.

    Also be very careful with any rags and the BLO is this type of oil will spontaneous combust given half a chance.


  2. maine_tyler | | #2


    Thanks for the response.
    Yes I do understand those things. The 'by the book' value of tangential shrinkage for Eastern Hemlock is 6.8%

    This is a bit of an experiment for me but there is much historical precedent for it. The beauty of b&b is that it can be restrained to a plane while not being restrained along its width. I would argue one of the best places for this hemlock to dry is in place on the wall. Perhaps a well controlled kiln with proper restraint is better, but the likely alternative on this project would be questionably stacked air-dry.
    I do think it's an advantage that it'll be up over a fall & winter before seeing the heat of summer, as I think this will slow the drying and prevent rapid drying stress cracks.

    I'll try to orient boards to 'bubble' rather than flare at the edges. I'm planning on using screws and don't mind returning for a second turning.

  3. user-7124595 | | #3

    Consider sealing the end grain to slow the drying, control the cupping. Heartwood should be out IMO

  4. RussMill | | #4

    First hand experience shows FAR MORE than 6.8% shrinkage

  5. [email protected] | | #5

    Hi Tyler,

    I have had great luck with using different variations of an oil mix on green wood, I make different mixes myself using various oils and thinners depending on the application. One tried and true old method is “boat soup” - 1/3 linseed, 1/3 turpentine, 1/3 pine tar. The pine tar acts as a fungicide. I would recommend potentially adding some form of fungicide to the mix if you are going to oil the face. I’ve also used tung oil (won’t grey or blacken with sun in the same way that linseed does), and added in various oil finishes to mixes to garner some of their properties (spar urethane, Sikkens, etc).
    I’ve used it on green white pine, ash, oak without any issues.

    I agree, board and batten, when installed correctly (no fasteners on the edges of the boards), works great for green boards. My assumption is that is a large part of its design.

  6. [email protected] | | #6

    One really great ready made product that I have used (though it’s rather spendy) is Heritage Natural Finishes. Their exterior formula has a fungicide/mildewcide in it, and I believe they will sell you just the fungicide/mildewcide if you want to add it to your own mix.

  7. maine_tyler | | #7

    Thanks Nick. That's great info. I was wondering about fungicide as I was reading that the oil can actually be a food source.
    Do you think it's really worth trying to get something on the back side? (bearing in mind that it's a rainscreen install) I'm definitely finishing the front once a bit drier...

    Heartwood out coincides with the grain orientation I was planning so that's good.

    "First hand experience shows FAR MORE than 6.8% shrinkage"
    Russell, what ballpark are you thinking its in? At, say, 10%, that's 1 full inch of shrinkage for a 10 inch board. That A LOT! I think somewhere around 5/8"-3/4" of shrinkage on a 10 inch board (close to 6.8%) would appear, visually, as a lot of shrinkage and may lead one to think that the percentage is quite a bit higher. I'd be interested in evidence of recorded shrinkage outside the documented values though. I'm sure its possible the growing conditions affect these values.
    (FWIW, western Hemlock is documented as having 7.9%)

    1. [email protected] | | #10

      Assuming you are saying you will use and oil mix on the face, I would finish all sides now, with a penetrating oil mix as described, and make sure it gets fully absorbed. It will slow down the shrinkage altogether, and manage cupping better than just finishing one side. I’d make one batch and use it on everything, with a fungicide/mildewcide.

      To me, 1” of shrinkage once fully dry, on board and batten, isn’t that big of a deal. 1/2” on either side, you’ll end up with 1”- 1 1/8” of gap, maximum, under each batten. Likely less. Use 2” battens and you’ve still got 7/16”-1/2” coverage on each board, minimum.
      And you’ll never see that gap anyway.

      I would preset the screws on the batten, bump them up tight to the previous boards, set them a bit deeper into the sheathing, bump up your next board tight to the screws, then set the screw, repeat on the next. Just be sure you don’t fasten any edges of the boards.

      1. [email protected] | | #11

        I would also be sure to finish the ends of all boards with something a thicker that will completely seal. I use spar urethane or a wax end grain sealer.

  8. Expert Member
    BILL WICHERS | | #8

    As an engineer, I'm very familiar with using values for things that come from tables in reference books. I can also tell you that wood is a natural product with a LOT of variation. I would NOT trust that book value for shrinkage unless you've tested a sample of EXACTLY the wood you're working with to verify things. Specific properties of wood vary with species, the region in which it was grown, and even how good the weather has been over the life of any particular tree.

    If you're using green lumber for siding, I'd put it up "shake style" with plenty of rough overlap so that any shrinkage is effectively hidden by the rough and random pattern, similar to a shake roof.

    Note that sometimes you get splits along with the shrinkage.


  9. Expert Member


    I've had a lot of luck covering wet wood with solid colour latex stains. They don't peel the way paint does. You would have to be careful not to get it where it would show.

    Think through the nailing pattern of the siding carefully so you don't get splitting as it shrinks, if you pin both sides of the boards.

  10. maine_tyler | | #12

    Good news is that I'm working with ballpark numbers given the design of b&b.

    My intuitive sense is that the number sounds about right. Looking at the math: 11/16" of shrinkage for a 10" board is indeed a lot, but still coincides with the 6.8% value. I'm having difficulty imagining a 10" wide board (specifically eastern hemlock) shrinking significantly enough to enter a ballpark where 1.5" would be lost, leaving a dried finish width of 8.5". That sounds like something I haven't dealt with.

    I agree with your sentiments Bill that a chart shouldn't be explicitly trusted given the natural variances of wood (engineers must love trying to assign values to such material!:), which is why I'm working in ballpark figures and not spacing my battens for exactly .68" coverage of the boards.

  11. Expert Member
    Akos | | #13

    I would 2nd the penetrating latex stains. I haven't done green wood, but have done a fair bit of pressure treated, which is not far off on moisture content, has held up much better than oil finishes.

    There are some paintable water repellent preservatives, I've had mixed results with them. Water based penetrating stain seems to hold up better.

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