A.Henri de Marne responds: All stained or painted wood siding will absorb water. Protective coatings help reduce absorption, but do not prevent it, especially after some weathering time. Plus, moisture is absorbed through shiplap joints, the tongues and grooves of solid-board siding, and the bottom edges of clapboards. Also, as any wood moves with the changes in the climate, thirsty, unpainted surfaces become exposed. Then there is capillary attraction between two meeting surfaces, which can drive moisture well beyond the exposed parts of any uncoated wood.

All these conditions make wood siding vulnerable to moisture. If it stays wet, it won’t be long before you have mildew problems. Once mildew sets in deeply, no surface power-washing, even with added chemicals, will reach deep enough to get rid of it entirely.

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