Black stains on limestone columns, pedestals- how to clean?

I’m starting a big job tomorrow and some of the mess consists of limestone features around several terraces. they are columns, pedestals, caps of retaining walls, etc. they have that black algae look. I know that stuff can get embedded deeply into limestone due to it’s porous nature. i also know that drilling it with high pressure can really beat up the surfaces. sorry, no pics yet, i can post some tomorrow.

any tips on how to lessen the appearance of the black discoloration? should i just downstream some house wash mix and then rinse at medium pressure?

i’m running a 4000 psi/4gpm vortexx gear drive machine (sure is a pretty machine.).

total, utter noob here. i’m still learning what tip degrees mean and how to adjust pressure on the machine, so go easy on me…:rolleyes:

I would so pass on this job. Limestone is one of the most finicky stones out there. It could clean up with housewash mix but probably won’t, it could clean up with safe restore, it may not clean up at all. Do you know what kind of limestone you are dealing with and if it is sealed or not? You can also damage limestone easily if you don’t know what you are doing and have to replace all the limestone. At best, you’re going to need heat and low pressure if chems don’t work and if the sealer is off of it. This is DEFINITELY not a newbie job and would pass on it till you get several years under your belt and understand chemicals. I even pass on some limestone jobs even now.

can’t really bail on it. we’re doing windows ans power washing all the paver patios and pool deck etc prior to a big party next week. i’m the go-to guy for the extended family, so i’m sort of obligated…

however, this is probably about an $8mil house. it’s fallen into a little bit of disrepair. none of this work has been done for years. i threw a big number at it and she went for it. i think i can tell her that “i’m not sure what i can do with the limestone, no guarantees, but i’ll do my best.” that will satisfy her. i don’t think her expectations are really high to begin with. it’s a beautiful house, but it’s also kind of a dump…

then i’ll go a safe route, house wash mix, low pressure rinse, and call it good.

does that sound like a good, low-risk plan?

House mix probably won’t touch it, so start off with roof mix if that doesn’t work. Then use low pressure (around 800 psi) WITH hot water if that doesn’t work, but that’s where you can get wand marks and make it look really bad if you’re not even with your wanding. Before I did anything, I wouldn’t do anything without a damage waiver saying you’re not held liable for any damage.

i talked to the homeowner today and explained that i couldn’t promise anything. i told her that i would do the best i could without damaging the surface. she was fine with it. the place is kind of messy, so any cleaning at all will be a dramatic improvement. i think she’ll be happy regardless.

i guess i’m not positive that this is limestone after all. i think it is, but maybe it’s just formed concrete?

here are some pics (dropbox links… still can’t attach images for some reason…_

yea… copper staining on the one… good luck…

not worried about the copper staining, customer knows that’s not coming off. mostly the black stuff.

yes but it can come off… if you knew what you were doing… or by referring them to a specialist…

ok. thanks. i’ll try to locate a specialist for the customer if she’s interested in cleaning the copper stains.

do you think trying a product like prosoco sure klean would be out of my league?

I have a masonry company I do a lot of work with… To be honest I generally send all of that type of work their way… That is a true specialty… And not something I want to get into… There is a whole science behind cleaning limestone the right way, and I have no interest in learning it. To many variables you really need to know what your doing.

just to wrap up this thread, here’s an update. we cleaned the limestone safely, and it turned out pretty good.

I used a mix of 3 gallons 12% sh, 1.5 gallons of water and 10 oz. simple cherry. downstreamed at 10 to 1 and let it dwell 15 minutes. sprayed it off with a 40 degree tip. no damage, no wand marks and significant improvement. thanks for the advice on being careful with this stuff.


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what is sh?


Sodium hypochlorite = Bleach.

funny coming back to read this thread after a few years. we’ve cleaned 10’s of $1000’s worth of limestone since then and it’s probably the most profitable, blow-our-customer’s-mind, collect-insane-before-after-photo, easiest cleaning we ever do.


Just reading this after I finished a massive job with basically all limestone walls. I managed to dull the black in many areas but still are certain parts of the wall (even after direct SH with a sprayer) it is still grayish. Any lucky with any other mixes? Is it possible to actually eradicate all of the black off a limestone wall? Not the best after photo but you get my drift