Sweep away loose dirt and other debris from around the spindles using a broom.
Fill a clean garden sprayer with 1 gallon of warm water. Add 1 cup trisodium phosphate and 1 cup chlorine bleach. Gently shake the container to mix the ingredients.
Spray a liberal coating of the cleaner onto the spindles. Let it sit for 10 to 15 minutes.
Spray down each spindle with water from a pressure washer. Keep the nozzle of the wand held at an angle and at least 12 inches from the wood. Move the wand back and forth over the spindles until they are clean. Keep the wand moving at all times.