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Community and Q&A

Priming the back of wood siding

Myrtleboone | Posted in GBA Pro Help on

I have heard that it is necessary to prime the back side of the spruce clapboard that I am installing on my house in order to prevent rot. I do plan on having a 1/2-3/4″ air space in between the siding and house wrap. Is this extra step necessary? Lots of labor and extra paint I’d rather do without. On the outer surface I will be sealing the knots with BIN oil based stain blocker prior to solid stain application. Thanks for your help.

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  1. Expert Member

    priming all six sides of your clapboard will both extend their lifespan and at least double the time until you need to repaint. I know it's a pain but well worth the effort.

  2. rockinroger | | #2

    I've always painted all 6 sides. My technique which you might find helpful, is as follows :
    I start with a pile on some sawhorses.
    Using a "slim Jim" , or "cigar", roller (6" - 3/4" nap) with an extension handle, a five gallon bucket, & screen, I paint all the end grains, tops, & bottoms first so I can stand them up to dry (if its not too windy) against a fence, or a gutter. To keep the gutter edge or fence top clean , I throw a drop cloth (a runner) over the fence, and some foil over the gutter if I don't have enough runners.
    I also put a runner on the ground so that I don't pick up any debris on the ends, or you can simply lay some of the siding boards down for that purpose.
    I start rolling off the top of the stack, one by one, and stand them up to dry, wet side against the gutter or fence. Then I come back, roll half way up in place, and by the time I get to the end they're usually dry enough to flip them over, & paint the part I couldn't reach. It goes pretty quick this way.
    While B-I-N is usually used for interior, if there aren't too many knot holes, I find it's cheaper, & faster to get a couple of cans of spray primer for this purpose. I use Binswanger 1-2-3 oil based primer, I find it more effective than Kilz. OOps! I just noticed you said you were going to stain them. Well, my mistake, but the process remains the same whether using stain or paint.
    I hope this helps.

  3. dankolbert | | #3

    Or get a sprayer.

  4. davidmeiland | | #4

    Or ask the lumberyard if they deal with a pre-stain facility and can get the siding pre-stained for you.

    The only thing I don't like about pre-staining is that the scraps become trash instead of kindling. In spite of that, we pre-stain 100% of siding and trim, and then further coat(s) after installed.

  5. Expert Member
    Dana Dorsett | | #5

    Not only does back-priming prevent rot, it prevents bowing & cupping. All siding takes on seasonal moisture from sources as diverse as direct rain, wicking, and vapor diffusion through the wall assembly. But if it's painted on only one side both the wetting rates and drying rates differ between the exterior & interior faces, which creates mechanical stresses that result in warping/bowing/cupping/splitting.

    But when you back-primer the siding the differences in drying & wetting rates are small, the goods stay pretty flat.

    At my own home a 91 year old 9" cedar clapboard (the bottom-most on the south side of a dormer) recently bowed to the point that it had popped & split through enough nails that a wind gust was able take it down. After some gluing/patching/filling/sanding/painting it will be going back up this weekend, but you can bet it's painted on all sides. When I first brought it down for re-work it had about 3" of sagittal depth over ~7' length, but with a couple of weeks in the basement to stabilize out of the sun in a stable humidity environment that bowing had been reduced to less than an inch, and won't be putting undue stress on the nails. Being the bottom clapboard it had the least protection from roof overhang, and at the bottom of the drain plane- clearly the piece with the largest moisture loads, and the biggest moisture swings. But that doesn't mean the others don't benefit from back-priming.

  6. wjrobinson | | #6

    Solid staining low cost knotty spruce siding with BIN for bleed thru is a lost cause. That's why clear vertical grain siding is sold, because it performs what you hope the low cost spruce will.

    If you don't want to see knots, don't buy knots.

    Back priming is best practice. Why do so if you are BIN-ing knots in low cost claps. Why stain at all. Just let all weather is an even less costly less laborious spec.

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