Fill a pump sprayer with 1 qt. chlorine bleach and 1 cup trisodium phosphate per gallon of warm water. Wear rubber gloves when mixing and handling the cleaning solution. Stir the solution with a long-handled wooden spoon or wooden stirring stick.
Prepare the pump sprayer according to the manufacturer's instructions.
Spray any plants surrounding the building with clean water from a garden hose to protect them from the cleaning solution. For extremely sensitive plants, cover them with a plastic sheet or tarp.
Spray the synthetic stucco with the pump sprayer to cover it evenly and thoroughly. Use a ladder to reach high places, if necessary. Mix more solution if needed.
Brush the synthetic stucco gently but thoroughly with a long-handled scrub brush. Start from the top and work your way down. Use a ladder if needed.
Rinse the synthetic stucco thoroughly with the garden hose, using high pressure. Start at the top and spray down to remove all traces of cleaning solution.
Rinse any plants around the home with water from the garden hose, or remove the tarp or plastic sheets.