Pressure washing--also called power washing--is a good way to clean a house exterior or prep it for a new paint job. Spraying water and, in some cases detergent, at high velocity, a pressure washer cleans dirt, grime, and chalk-like residue from deteriorating paint from the surfaces of siding and trim. This is an excellent first step to ensure that a new coat of paint will last.

A pressure washer is not meant to be used for removing paint from wood siding or brick--holding a strong spray in one place long enough to remove paint can erode soft wood grain or dislodge mortar from between bricks. Though it can be used on most siding materials, including wood, vinyl, metal, and some types of masonry, a pressure washer should not be used on hardboard siding because moisture is hardboard's worst enemy and it shouldn't be used on stucco because it can ruin the material. Also, do not pressure wash your house if you suspect that it may be been painted with lead paint prior to 1978 (contact your public health department or the EPA at 800-424-LEAD). If your home is two-stories or very large, it's safer to let a professional pressure-washer do this job for you.

You can rent a pressure washer from most rental equipment dealers for about $50 to $75 a day. They come in various sizes for different types of jobs, from about 1200 to 3000 psi (pounds per square inch, the power of the spray). A 1200 to 1500 psi model is safer to use with vulnerable materials such as wood, aluminum or steel; you can use a stronger 2500-3000 psi model for a material that doesn't have a painted surface, such as vinyl. The stronger types work faster. Choose one that will allow mixing detergent into the spray. Plan to mix about 1 pound of TSP (or a non-phosphate substitute) into every 4 gallons of water for the washing (but rinse with water only).

Scrub any mildew from the surface before power washing.

Use both hands when operating the equipment and never use it while standing on a ladder. For high areas, also rent a 6- to 12-foot adjustable extension shaft, but be aware that this can be a bit tricky to handle. Start with the nozzle about 3 feet from the wall and work your way closer until you feel the spray is strong enough--but not too strong. Never get closer than 12 inches from the siding or you may damage it; do not point it at windows and avoid aiming it upward, where it might drive water under siding. Remember: this is extremely high pressure, so never point the nozzle at anyone and be sure to wear safety glasses. Also keep the wand, the water, and your self away from any electrical wires or devices.

Here are the steps to follow:

1) Protect any plants, light fixtures, and any breakable materials around the house with plastic bags, duct tape, and drop cloths. Move obstacles and outdoor furniture away from the house.

2) Connect the pressure washer to a garden hose. If the machine has a detergent dispenser, fill it with a cleaning solution (a mixture of water and TSP (or a non-phosphate detergent substitute).

3) Working from the top down, hold the wand at about a 45 degree angle, and spray soffits, overhangs, gutters, and downspouts. Then move it across the siding from side to side at a steady pace. Hold the nozzle a couple of feet from the wall and adjust its distance back and forth for the best cleaning action. As well as you can, direct the water slightly downward so it isn't blasted up under horizontal lap joints of the siding. When working near windows, direct the nozzle away from the window, holding the wand at an angle so you don't drive water into the window or frame.

4) Rinse off the siding with clear water, using a garden hose--work from the top down. If you plan to paint, allow the siding to dry for at least two days.

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