How to Clean House Siding Before Painting

Siding acts as the first line of defense for your home, but over time, it collects layers of organic growth, road grime, and oxidized paint residue that will ruin a fresh coat if left behind. Painting over a dirty surface is essentially trying to glue new paint to a layer of dust; the result is premature peeling and a wasted weekend of labor. Preparation is where the longevity of your paint job is won or lost. Cleaning siding effectively doesn't mean just blasting it with water; it requires removing the microscopic film that prevents primers from bonding. When done well, your siding will look clean, feel dull rather than powdery to the touch, and be completely free of active mold or mildew.

  1. Shield Plants and Power First. Cover all nearby plants, shrubs, and flowers with plastic drop cloths to shield them from cleaning chemicals. Turn off the main electrical power to any outdoor outlets in the work area and seal them with painter's tape.
  2. Mix Your Cleaning Weapon. Fill a large bucket with a mixture of one part bleach to four parts water, or use a specialized siding cleaner formulated for pressure washers. For heavy mildew, add a cup of trisodium phosphate (TSP) to the solution to help break down the grime.
  3. Saturate Bottom to Top. Working from the bottom up, spray your cleaning solution onto the siding using a garden sprayer or a low-pressure setting on your power washer. Let the solution dwell on the surface for 10 to 15 minutes to break down the dirt.
  4. Attack Stubborn Grime Spots. Use a long-handled, soft-to-medium-bristle brush to scrub areas with heavy dirt or mildew buildup. Focus on corners, eaves, and undersides of lap siding where debris typically accumulates.
  5. Blast Away Every Last Residue. Rinse the siding using clean water, working from the top down this time to ensure all dirt and detergent are washed away. Ensure you flush all crevices so no chemical residue remains.
  6. Wait for Complete Dryness. Inspect the siding for any missed spots or remaining chalky residue. Allow the siding to dry completely for at least 48 hours before applying any primer or paint.