Over time, stucco can appear dirty or stained as it is constantly exposed to the elements. There are many options available for cleaning stucco, but stucco can easily be damaged by certain cleaning methods. Bleach is one choice for cleaning stucco walls, but it should be used very carefully to avoid damaging the stucco.
Before applying bleach to stucco, apply a diluted bleach mixture of 1/4 cup of bleach to a one-quart bucket of water. Apply small patches of this mixture to a hidden and inconspicuous patch of your stucco wall to determine if the stucco is in any way damaged or lightened by this diluted bleach mixture. If the wall is affected, you can try diluting the mixture even further with more water. Otherwise, if the wall shows no signs of damage, the bleach will not damage your stucco house.
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A stiff scrub brush along with a diluted bleach and water mixture can be used to clean stucco stains, but always start out using the brush with light pressure to avoid damaging or crumbling the stucco. A pressure washer may be used to rinse the bleach mixture off of the stucco, but it should be used on the lightest setting available to prevent damaging the stucco material.
The most common type of stain found on stucco walls is called efflorescence, which is actually minerals and salts inside of the stucco leeching out onto the surface. This type of stain does not require bleach and can be removed with vinegar and water.
Synthetic stucco is a foam base coated in a stucco-like material to give the appearance of stucco but with further insulating properties. This material is not stucco at all, and bleach should not be applied to it because of the delicate nature of the foam and the stucco-like top coat. Special cleaners are available for synthetic stucco stains.