How to Prep Siding for Exterior Staining in Boston

Step 1: Remove Mildew and Graying

Boston's humidity creates ideal conditions for mildew growth on north-facing and shaded siding surfaces. Gray coloration on cedar and pine siding is caused by UV-degraded wood fibers and mildew, not just weathering. Before any stain is applied, all mildew and gray must be removed with an oxalic acid-based wood cleaner. This step is non-negotiable — stain applied over mildew traps the contamination and shortens the application's lifespan significantly. Cabot wood cleaner products →

Step 2: Pressure Washing

After applying wood cleaner, we pressure wash the siding to remove the cleaner along with the loosened mildew, gray, and any other surface contamination. The key is using appropriate pressure — high enough to clean effectively, low enough not to raise wood grain or damage siding. Soft wash technique (lower pressure, appropriate nozzle) is appropriate for most wood siding. After washing, the siding needs 48–72 hours of dry weather before any stain application.

Step 3: Moisture Testing

Wood siding must be below 15% moisture content before stain application. We use a moisture meter to test multiple locations before starting — particularly at north-facing walls and areas that were slow to dry after washing. Staining wet wood is one of the most common causes of premature stain failure. The stain sits on the wet wood surface rather than penetrating, dries as a surface film, and peels when the wood dries out beneath it. TWP application requirements →

Step 4: Sanding Rough Areas

Any rough, raised grain, or weathered areas that didn't smooth out from washing get hand-sanded before staining. Raised grain absorbs excessive amounts of stain and can cause blotching — particularly visible on semi-transparent stains where grain variation is visible. Sanding with 80–100 grit along the grain direction smooths raised fibers without removing too much surface wood.

Step 5: Application Timing and Conditions

Temperature during application and for 24 hours after matters for stain penetration. Oil-based stains require above 50°F air and surface temperatures. Application in direct sunlight on hot days causes the stain to skin over on the surface before penetrating — we work in shade or on overcast days when possible. Two thin coats penetrate better than one heavy coat; we apply the first coat, let it penetrate for 20–30 minutes, and back-brush to work excess stain into the wood before it skins. Armstrong Clark application guide →