How to best clean stucco?


Old 05-15-20, 10:24 AM
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How to best clean stucco?

I have a 20 year old white stucco home that has some discoloring (dark/black) in some areas like the chimney. Some quick googling tells me its not a good idea to power wash stucco? Saw solutions using simple green, or borax. Is there a recommended method?
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Old 05-15-20, 10:47 AM
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You need to make sure you have true stucco (Portland cement plaster) about an inch thick applied over woven wire lath or diamond mesh lath. I don't see how power washing could do much harm to this. The worst is that it could take off some finish. I am guessing that the stucco has not been painted. If it has been painted it could take off some paint. If the paint is that loose it probably needs to come off anyway. There may be some risk of pusing water through the cracks in the stucco. The underlayment, if properly installed, should allow for drainage without damge to the sheathing or insulation.
Don't use too concentrated a stream. A pointed stream could cut through the stucco. You want it clean not taken off.
Unless you own a power washer I would try scrubbing it first. The cause of the stain will determine what works. If it is organic borax or Oxiclean might work. Ammonia will work for some things. Chlorine bleach will work for some things and vinegar or very dilute muratic acis will work for some things. Try small sections first and RINSE BEFORE AND EXTREMELY WELL AFTER, ESPECIALLY IF YOU USE ACID.
If you have real three coat stucco it is hard to harm it. Just try small and learn what works and rinse before and after.

BUT IF YOU HAVE SYNTHETIC STUCCO, a thin coat of cementitious acrylic modified or acrylic based material applied over insulating foam. be extra careful, You could cut right through the coatings and into the foam

Now if you go the less radical route and use a cleaner there are a couple important things to know. First is start from the bottom and get everything wet with water first. If you start at the top the stains may indeed clean off but the dry stucco below will absorb it into the pores of the finish and it will penetrate and not come out. This is not intuitive but makes sense if you thing about it
Old 05-15-20, 12:46 PM
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I've pressure washed countless stucco houses getting them ready to repaint. Using a cleaner to do the bulk of the work is always best. You always want to use care PWing around any openings [windows, doors, vents, etc] Stubborn stains/grime usually benefit from scrubbing. I like an old broom for the added reach.
Old 05-16-20, 08:45 AM
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ok thanks all. great feedback. I definitely have real stucco for sure. I'll try a cleaner first and see what I can remove. Then pressure wash on lowest setting and work up, slowly increasing the power. Its not alot of areas and we are expecting we'll need to replace it all at some point. So many homes in our neighborhood have had issues and have done replacements. I've had mine checked and so far no damage.
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