What is the recommended substrate moisture content for painting or priming bare wood (Douglas Fir - rough cut T1-11)?
I figured it out. 12% is ideal (a few percent less for dry climates, and a few more for wet climates). Never paint on anything with 20% or more.
I've worked on several homes with bad flaking paint. One time I thoroughly scraped and let dry in hot weather for a week, and used high quality primer and paint, only to be scraping flaking paint again in 4 years.
I've consulted several sources on the problem and gotten mixed advice. Some said that it was inherent in wood siding, but I've seen 15+ year old painted siding in the same weather conditions without a single flake. Someone else said that it was a bad first paint job, and the siding is water logged, and that once the siding is water logged, it's ruined and will have to be repainted every 3-5 years. The most persuasive and logical answer that I found, in my opinion, is that some previous paint job was done with too high a moisture content in the wood, and every subsequent paint job will suffer from a slowly (3-5 yr at a time) peeling problem. The only way to fix it without replacing the siding is to remove all the paint with stripper and/or pressure washing and allow to dry to a proper moisture content before priming.
Visual & touch dry. I've never measured the moisture content of anything & just wait a day or 2 for it to dry & not a single moisture content issue in decades. From what I understand Precise Moisture Content is really only needed for building furniture & cabinetry or installing flooring & interior trimwork.
Basically, if it's Interior Meant it will dry out to bone dry & you want to start as close to that as possible so things don't do much more or any other moving & twitching. If Exterior Meant it'll be exposed to humidity & seepage repeatedly & should be allowed to do that as freely & naturally as possible.
This is why Exterior items are typically just Weathering Surface treated with Primers, Paints & Polyurethanes. Not allowing the wood or wood-based product to ever fully dry means there are no breaking points & the wood product stays plump & flexible. It won't overly constrict nor overly expand & moves with humidity changes in a range it spent its life in.
Single-coat Priming all 6 sides of something is an accepted practice & has shown no pre-mature detrimental effects, since the coverings breakdown quite quickly & long before wood tires-out. But, covering & re-covering that Primer & especially with Paint or encapsulating the wood product in a Polyurethane have shown very pre-mature failures & instabilities.