Mildew or mold on exterior paint — San Diego, California

Dark green, gray, or black splotches on exterior-painted surfaces — siding, trim, soffits, and window frames — are usually mildew or mold growth on the paint film itself. Mildew thrives on north-facing walls, shaded areas, and surfaces near sprinklers or dense landscaping where moisture lingers. Unlike dirt, mildew is organic growth that feeds on the paint's binder and won't wash off with water alone. A bleach solution test confirms mildew: apply a few drops of household bleach (1:10 dilution) — if the spot lightens within 1–2 minutes, it's mildew; if not, it's dirt or staining. Professional pressure washing with mildewcide ($200–$600 for a typical home) removes existing growth, but repainting with mildew-resistant paint ($40–$60/gallon) is needed if the paint film is degraded. Prevention: trim vegetation 12–18 inches from walls, redirect sprinklers, ensure gutters don't overflow onto siding, and use paint with built-in mildewcide for shaded exposures.