OR NOT BACK PRIMING
Back priming for those of our readers who don't know is normally applied to exterior wood siding. humidity that appears on a houses sheathing (second wood layer of skin) from popping the paint off of the outside of your home.
When a houses sheathing has a moisture barrier (such as Tyvek) and the siding is primed on its back surface, trapped moisture can occur between the primed siding and the Tyvek.
Since the moisture is trapped it will dry out over time. Once dry, moisture of course can no longer rot the wood, create mold or have a chance of going through the non-back primed siding.
Moisture wicking through raw siding would cause the surface paint to crack, lift and peel.
Basically, siding with a primed backing and Tyvek should create a (hopefully impenetrable) sandwich like moisture barrier.
So, in the case of exterior siding, back priming would depend on the type of your interior/exterior environment and how it varies from season to season.
You would then need to consider if your said environmental conditions would require a sandwich type weather barrier to trap any excess mold/moisture build up.
SHOULD INTERIOR WAINSCOTING
BE BACK PRIMED
Now the question is “should interior wainscoting be primed on its back side”? Well, like in the above siding example it would depend on the situation. Lets say that the wainscoting is in a bathroom (or damp basement) and applied over drywall (Sheetrock).
If humidity or moisture gets between the drywall and wainscoting mold could build up on the drywall. Especially if the drywall is unprimed/painted and not specifically made for high humidity surfaces (Eg. Aqua board or some fiberboards).
Regular non- fiberboard type wainscoting that's not back-primed could also become moldy. Primed wainscoting for that matter (front or back) can mold if a mold resistant primer is not used.
On the other hand, when wainscoting is nailed directly to untreated or unprotected wood (Eg. 2X4s, 2x6s, or sheathing) and the wainscoting is fiberboard or back primed, then the 2x4 stud framing and sheathing could begin to rot and mold but not the wainscoting.
Unprimed, rotted or moldy wainscoting is (usually) easier to see and replace as oppose to 2x4/6 wall studs or sheathing.
I, myself, may prime the back of wainscoting if I'm thinking that humidity/mold might be a factor, especially in the case of MDF board. MDF tends to easily absorb moisture/mold and can degrade quickly.
Fiberboard MDF is in most cases way more water resistant than regular MDF though. If I where using fiberboard MDF I would make sure to follow the manufacturers recommendations as to whether the board needs back priming or not.
I would also keep in mind that properly installed barrier/s along with the correct type of barrier itself would need to be in place prior to priming/painting.
If said barrier/s are not or can not be put into place and there is some doubt as to what might happen then wainscoting should not be used.
What I'm saying, I guess, is that it might be a better choice not to use wainscoting in humid rooms in the first place.
With areas or rooms where moisture is not a problem then back priming is, in my opinion, unnecessary.
Others, including your local home improvement center sales rep may correctly disagree with me and have reasons for or against back priming depending on a specific geographic area or certain kind of project at hand.
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