Black stains on stucco

Residential
1

I’d like to start by saying that I spend A LOT of time on here reading through all the threads and yes I have and do use the search function…

I came across 2 particular jobs a few days ago that gave me a really hard time. As you can see in the pictures, I was not successful at removing the stains.

  1. I DS (hi draw DS Injector) 10% SH from my chem tank, full SH and Elemonator - no water. Let it dwell for 10-15 mins and rinsed. Did that about 3 times without any significant results.

2.Brought out the x-jet, filled a 5 gallon bucket with 3 gallons SH & 2 gallons water and some Elemonator. Let it dwell for 5-10 mins and used a soft brush to do a bit of scrubbing. There was some improvement but nothing significant.

  1. Pump Sprayer - Filled it with half SH half water, sprayed, scrubed with soft brush again, dwell, rinsed. Did that a few times and that’s about as good as it got.

  2. Started to dip my brush in the straight SH chem bucket and doing more scrubbing, not a whole lot of difference.

  3. I also tried a bit of oxalic (my own mix, ran out of f9), I don’t think it made significant difference.

I noticed a few things -
The homes where I did the side and the front, the stucco finish was very porous, I had never dealt with that stucco finish before. Also, they had A LOT of cracks. I also did the balconies of one of them and noticed a ton of cracks on top of the balcony “molding” stucco if that makes sense, and water was definitely getting in there and flushing out (dripping) black stuff from in there. Not sure if this had something to do with it or not. I was able to remove a good amount of black streaks from the front of that one house, but still not enough to make it look good.

This is my first time running into something like this. I do stucco all the time and 9/10 everything comes off just fine with just down streaming (I know that @squidskc always says you can’t DS stucco, but i get really good results most of the time. and I know @Innocentbystander says you should walk away from stucco, but 9/10 homes in my area have stucco so I may as well just close shop all together) .

P.S
Couldn’t use a stiffer brush bc I did a small test spot with it and the paint was coming right off the stucco.

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2

Try muriatic acid on a sprayer, test it on a corner.
No dilution

3

I ran into the problem as well, I just talked to the owner about it and they were happy with the results as it was. Luckily it wasn’t so large of a spot. It was mainly under windows. I wasn’t happy with it, so I’m interested in suggestions as well. Looking very closely it almost appeared to be dirt trapped in the very porous stucco. I also feel like I just should apply more pressure to get the trapped dirt out but I’m just very cautious when it comes to pressure on stucco.

I’m nervous about using muratic acid. But I will try it next time in a inconspicuous area.

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4

Ive had a few that didnt get 100% clean. I told them it may not come off 100% by HW and spraying alone. They were happy with 90% gone and didnt want to pay more for me to try for 100%

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5

You might have just needed a stronger solution and more dwell time. I’ve had a few stucco jobs that took straight sh in a pump up, scrubbing, multiple applications, and pressure. If ours real stucco it can take some pressure. Dry vit is much more fragile.

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6

Like @bluewave18ft said sometimes it’s just set in.

If two passes and a soft brush don’t tackle it it’s baked in the cake. Not much you can do about it.

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7

Yeah, she was pretty happy with it and did acknowledge there was a significant improvement. I just sorta wish I would’ve known prior, and had been able to set her expectations.

8

Gotcha, I’ll try straight SH next time, but at the same time, I’d rather walk away from jobs like these in the future… too much time effort for not enough $$$.