Because detergent needs extended contact to work effectively, it can be applied only when the machine is in its low-pressure mode. Detergents can pose a problem, however, especially if they contain bleach. Of course, if your siding is only a little dusty, it may not need a detergent.
If some of your siding has begun to grow mildew, however, a little bleach is required. Using bleach on siding is tricky, so start with as little as possible, for as short a time as possible.
For a larger mildew problem, you might try Jomax, available in paint stores (Zehrung, Chempro Division, 16416 S.W. 72nd Ave., Portland, OR 97224). This mildewcide concentrate requires you to add bleach to the solution, but it deactivates the bleach after it’s applied. Spray it on sparingly with a garden sprayer.
With the mildew problem handled, you’re ready to wash the rest of the siding. Next, set the detergent container next to the washer and slide the open end of the plastic siphon tube over the brass nipple on the underside of the pump. Feed the filtered end of the tube into the detergent container.
With the water to the pump turned on, start the gas motor and, within a minute of starting, begin to spray an area of the house with detergent, working from the bottom up. You won’t want the detergent to dry on the siding before you can wash it off, so keep the size of the area manageable. Allow the detergent to work on the siding for at least 2 to 3 minutes.
Working from the top down, rinse the siding thoroughly. If you see spots that won’t come clean, try moving the nozzle a little closer to focus the pressure. Watch the siding closely.