What wood moisture content is safe for painting exterior surfaces in New Brunswick’s humid climate?

Wood must be at or below 15% moisture content before you apply any primer or paint to exterior surfaces in New Brunswick. This is not a guideline — it's a hard threshold. Paint applied over wood with higher moisture content traps that moisture inside, and as the wood dries and the trapped water tries to escape, it lifts and blisters the paint from underneath.

Wood moisture content is measured with an inexpensive pin-type or pinless moisture meter, which you can find at hardware stores in Moncton, Fredericton, and Saint John for about 0-80. Professional painters use them routinely before exterior projects. For a pin-type meter, you press two small probes into the wood surface and read the percentage on the display. Pinless meters use electromagnetic sensing and don't leave marks — useful for finished surfaces. Either type works fine for checking siding, trim, and decking before painting.

New Brunswick's climate creates specific moisture challenges that make this measurement particularly important. Spring is the most dangerous time. Snowmelt in April and May saturates soil and drives ground moisture upward through foundations and lower siding courses. Spring rains are frequent and heavy. Wood siding that has been exposed to a NB winter often comes out of the season holding 18-25% moisture or higher — far above the painting threshold — even if it looks and feels dry to the touch on a sunny May day. Painting too early in spring is one of the most common causes of exterior paint failure in the province. Check moisture levels in multiple spots, including on the shaded north side and in lower courses near the foundation, before proceeding.

After any rain or pressure washing, wait at least 48-72 hours of dry, sunny weather before testing moisture content again. Shaded areas, north walls, and wood siding near the ground take longer to dry than south-facing sun-exposed areas. A good rule of thumb: if you wouldn't be confident the wood has been consistently dry for three or more days, test before you prime.

The 15% threshold applies to the wood beneath any existing paint as well. On surfaces where the existing paint is intact and well-adhered, moisture in the substrate is less of a concern because the existing paint film is already acting as a barrier. Where the old paint has peeled, cracked, or been scraped away exposing bare wood, those bare patches need to reach below 15% before spot-priming and repainting.

Temperature also interacts with moisture. On cool NB mornings, dew and condensation can wet the wood surface even on an otherwise dry day. Avoid painting before 10 AM or after about 4 PM in the shoulder seasons — let the morning dew burn off fully. The ideal painting conditions in NB are mid-morning to early afternoon on a day with low humidity (below 60-65%), air temperature between 15-25°C, and no rain forecast for at least 24-48 hours.

For homeowners who want to start their exterior project as early as possible in spring, the practical answer is: be patient. A week of dry, warm weather in late May is worth waiting for. Rushing the job before the wood is ready adds another repainting project three years from now — and in NB's short exterior season, that's a real cost.

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