Stucco is a delicate surface with a texture that can be easily damaged by improper pressure washing. Damaged or flaking areas are particularly vulnerable to washing with high pressure, and dirt and grime can get into the crevices, where it's hard for the water to reach. Cleaning stucco requires more than just knowing what PSI to use, though generally a setting between 1,500 and 2,500 PSI is appropriate for stucco. With some know-how, the right supplies and a bit of effort, your stucco can look like new.
As preparation for the pressure washing, it's best to patch any cracks or gaps in the stucco. Without the patches, the water can be forced in through the cracks, behind the stucco and into your walls. Patching existing stucco isn't difficult, and there are many pre-mixed stucco patches commercially available. Matching the old and new stucco can be difficult, but as it weathers it will blend in to the existing stucco. Allow the patches to dry before your pressure wash, which generally takes a week or more. Follow the directions of the particular stucco patch you've chosen.
Detergents will make your job a lot easier. These should be applied using a pump-up spray can, or a pressure washer with detergent dispenser, but at a very low PSI. Since the point is just to get the detergent on the stucco so that they can work, a spray can is the easiest option. Apply the detergent one section at a time, working from the bottom of the surface up to the top. Let the detergent sit for five or 10 minutes, but don't let it dry. If it starts to dry out, re-wet it with the spray can, a garden hose or your pressure washer (still on a very low PSI). Detergents can be harmful to plants and landscaping, so cover any nearby areas you don't want the detergent to touch.
Set your pressure washer at a setting of between 1,500 and 2,500 PSI and test it on an inconspicuous area of the stucco before you use it on your surface. This will let you know if the PSI needs to be adjusted down without damaging visible areas. Keep the tip of the pressure washer about 2 to 2 ½ feet away from the wall and hold the wand so that the water stream is angled, hitting the stucco at around a 45 degree angle to avoid damage. Rinse the detergent away, working in sections and moving from the top down. Work around your windows, as a pressure washer may damage them.
Some stains may need extra scrubbing. Rust stains, for example, will need more than the detergent and rinse to get the job done. Use a cleaner meant specifically for your stain and scrub with a soft scrub brush before using the pressure washer or a garden hose to clean away the rust remover and any remain bits of the loosened stain.