TL;DR
Stucco should not be pressure washed. It is a porous cement finish coat that erodes under a concentrated stream, and synthetic stucco systems are a thin coating over foam board where a breach lets water into the wall assembly. The correct method is a mild detergent at low pressure, dwell time, a soft brush, and a gentle rinse.
Why stucco fails under pressure
Stucco looks like masonry and behaves nothing like it.
The finish coat is a thin, porous layer of cement, sand, and lime applied over a base. It is designed to shed rain running down a wall, not to resist a stream driven at it.
A pressure stream does three things to it. It erodes the sand-textured surface unevenly, leaving light patches wherever the wand lingered. It widens hairline cracks that were previously cosmetic into paths that carry water. And it drives water into the porous body of the material, which then has to dry from the inside.
The blotchy result is the part homeowners regret most, because a stucco wall cannot be spot-repaired invisibly. Patching a gouged area means matching texture and color across a whole elevation.
Source: stucco and plaster industry maintenance guidance
Traditional stucco and synthetic stucco are different problems
Traditional stucco is a cement-based system applied in coats over lath, usually on a solid substrate. It is thicker, harder, and more forgiving than the synthetic version, but it is still porous and still erodes.
Synthetic stucco, sometimes described as an exterior insulation and finish system, is a thin acrylic-based coating over foam insulation board. The coating is the entire weather barrier, and it is only millimeters thick.
Puncturing or eroding that coating on a synthetic system is not a cosmetic problem. Water that gets past it reaches foam and sheathing that dry very slowly, and moisture problems inside those assemblies are expensive and hard to detect.
How to tell them apart: tap the wall. Traditional stucco sounds solid and hard. A synthetic system sounds hollow and gives slightly under a knuckle. If you are not certain which you have, treat it as synthetic and use the gentlest method.
The method that works
1. Prep. Close windows and doors, cut power at the breaker to exterior fixtures, cover what you cannot disconnect, and move furniture away from the wall.
2. Soak the plants. Wet beds and shrubs with plain water and keep them wet, then rinse them again at the end.
3. Pre-wet the wall, top down, gently. Stucco is porous, and a dry wall will absorb cleaner unevenly and streak. Wetting it first is not optional here.
4. Apply a mild cleaner, bottom up. A mild detergent, or a diluted bleach-based exterior wash for mildew, from a pump sprayer. Work in sections small enough to return to before they dry.
5. Dwell. Five to ten minutes, out of direct sun, never to dryness. Mist with plain water if a section starts drying.
6. Soft brush the stubborn areas. A soft-bristle brush, worked in a circular motion, light pressure. Never a wire brush, never an abrasive pad, and never hard scrubbing that will burnish the texture.
7. Rinse, top down, wide and gentle. A garden hose is enough for most walls. If a pressure washer is used, it is a wide tip at the lowest setting held several feet back -- a rinse, not a clean.
8. Rinse the surroundings and let the wall dry fully.
The general chemistry-first approach is in soft washing and when it applies.
Common stucco problems and what they need
Mildew and algae on shaded elevations respond to a diluted bleach-based exterior wash with proper dwell time. This is the most common complaint and the easiest to fix.
Efflorescence, the white powdery bloom of mineral salts, comes from moisture moving through the wall. Brushing it off dry works, but if it keeps returning the real issue is water getting into the assembly, which is a repair question rather than a cleaning one.
Rust streaks below fasteners or fixtures need a rust-specific product. Never apply one over or under a bleach-based cleaner -- acids and bleach together release chlorine gas. Rinse thoroughly between products.
Cracks should be documented, not washed harder. Hairline cracking is normal in stucco; anything wider, or cracking in a stair-step pattern, is worth a look from a stucco contractor before the wall gets wet.
Staining below windows and joints often means a flashing or sealant problem upstream. Cleaning it removes the evidence without fixing the cause.
What not to do
Do not use a narrow tip on stucco under any circumstances. Do not hold a wand close, even with a wide tip. Do not use a turbo or rotary nozzle.
Do not use acidic masonry cleaners on traditional stucco without testing, since acids attack cementitious materials, and do not use them on a synthetic system at all.
Do not scrub with anything stiff. Stucco texture is easy to flatten and impossible to restore in a patch.
Do not work from a ladder with a wand. The recoil is a fall hazard and there is nothing to brace against; use an extension pole from the ground or hire the height out.
The wider list of materials that fail under pressure is in surfaces you should never pressure wash, and material-by-material tolerances are in what PSI each surface can take.
When to hand it over
Two-story elevations, walls with widespread cracking or visible staining patterns that suggest a moisture path, and any synthetic system you are not confident about are all better handled by someone who cleans stucco regularly.
The same is true if the wall is painted. Painted stucco adds a coating failure mode on top of the substrate one, and testing adhesion before washing matters as much as it does on wood.
If your home mixes stucco with masonry, note that mortar joints have their own low tolerance, covered in cleaning brick without blowing out the mortar.
FAQ
Can you pressure wash stucco?
No, not at meaningful pressure. Stucco is a porous cement finish coat that erodes under a concentrated stream, leaving permanently blotchy patches and widening hairline cracks. A pressure washer may be used at the lowest setting with a wide tip held several feet back, purely as a rinse.
How do you clean stucco without damaging it?
Pre-wet the wall, apply a mild detergent from a pump sprayer working bottom up, let it dwell five to ten minutes out of direct sun, brush gently with a soft-bristle brush, then rinse top down with a garden hose. Rinse surrounding plants before and after.
What is the difference between stucco and synthetic stucco?
Traditional stucco is a thick cement-based system over lath and sounds solid when tapped. Synthetic stucco is a thin acrylic coating over foam insulation board and sounds hollow. The synthetic version is far more vulnerable, because its thin coating is the entire weather barrier for the wall.
How do I remove mildew from stucco?
Apply a diluted bleach-based exterior wash at low pressure, allow several minutes of contact time without letting it dry, brush gently if needed, and rinse thoroughly from the top down. Pre-wet plants below and rinse them afterward, and never combine it with an acidic cleaner.
What is the white residue on my stucco?
Usually efflorescence -- mineral salts carried to the surface by moisture moving through the wall. It often brushes off dry. If it keeps coming back, the underlying issue is water entering the assembly, which needs a stucco or moisture contractor rather than repeated cleaning.
Stucco is the surface where the cheapest possible mistake and the most expensive possible repair sit closest together. Our pressure washing service treats stucco as a soft wash surface, never a pressure one. Get a quote and we will look at the walls first.