Introduction
I’m Paul from Trusted House Painter, and today I’m going to show you one simple—but crucial—step most homeowners and many DIYers skip: checking the moisture content of wood before you paint it. Painting over wet wood is one of the fastest ways to guarantee paint failure. In this post I’ll walk you through why moisture matters, how to use a moisture meter, what readings to watch for, and a few on-the-job tips I use when prepping fascia, siding and other exterior wood.
Why moisture matters
When you prime or paint a wet surface the trapped water turns to vapor as it evaporates. That vapor has to escape through the paint film, which often causes the coating to blister, peel or fail prematurely. Painting wet wood defeats the whole point of protecting your home with paint.
Common cause: missing shingles and exposed end grain
One very common problem I see: shingles that don’t extend far enough to cover the fascia or ends of boards. When a shingle is missing at the edge, water runs down the roof and soaks into the end grain—end grain absorbs water quickly and becomes a moisture trap.
Tip: whenever possible have shingles cover the end of the fascia. It will preserve the wood and save you a lot of prep work later—especially on older houses with decorative fascia ends you don’t want to cut off.
How to use a moisture meter (step-by-step)
- Choose a reliable moisture meter designed for wood.
- Take multiple readings across the piece of wood—top, middle and end grain—because moisture varies across a board.
- Compare readings: pressing harder or testing different spots will produce different numbers, so test gently and consistently.
- If a reading is above the acceptable threshold, wait and let the wood dry before priming or painting.
In the field I check several spots. In one demo I did, a quick poke of my finger (not gouging, just pressing) made the meter spike to about 30% moisture, illustrating how much pressure and location can change a reading.
Then I stuck the meter into the wood near the center and got right around 15%, which is the general upper limit we use for painting. At the end grain it read about 11%, and at the very top it was under 10%. Those lower numbers are safe to prime and paint.
Acceptable moisture levels for painting
- Under 10–15%: Ideal for exterior wood painting and priming.
- 10–15%: Acceptable—use caution and confirm conditions are stable (no imminent rain).
- Above 15%: Hold off on painting and let the wood dry out.
- Above ~20%: Definitely too wet—sand, dry, and recheck before coating.
What I do on a job
If I find moisture above the acceptable level (like >20%), I palm-sand the area to remove any compromised primer or contaminated surface, then let it dry thoroughly. If the weather is cooperative, I’ll come back the next day, re-check moisture, sand again if needed and prime with a quality wood primer once readings are under the threshold.
Other surfaces to check
Moisture meters aren’t just for wood—use them on stucco, fiber cement (Hardie board) and other substrates when you’re unsure about dampness. The concept is the same: never coat a substrate that still contains moisture that can vaporize and damage the coating.
Pro tips from the jobsite
- Always take several readings across an area—don’t trust a single spot-check.
- Watch end grain closely; it soaks up moisture faster than face grain.
- If your painter won’t use a moisture meter, that’s a red flag—real professionals check moisture before starting a job.
- Plan exterior painting for stable weather windows—shoulder seasons can be unpredictable.
Use a moisture reader. Ask your painter to use a moisture reader. Make sure it’s under 15% moisture content—and if it is, you’re golden.
Conclusion
Checking moisture is a small step that prevents big problems. Before you prime or paint any exterior wood, use a moisture meter and verify readings are below about 15%. If you’re hiring someone, ask them to check moisture—it’s a simple sign they’re doing the job right.
If you want to learn more about moisture meters, acceptable levels for different materials, and common mistakes to avoid, there are deeper guides and full articles available from Trusted House Painter.
