Black mold on limestone

Hello everyone! I would like to get some help and recommendations on what type of solutions to use to get rid of this black mold. My power washer runs on cold water, it would be a lot better to have a hot water one I know. Thanks for y’alls help!

Hot water doesn’t matter on that. This is my bread and butter. Already done over 200K sq ft of this this year.

Use your washer to clean the crap off the surface. Then direct application of chem (either pump up or 12v). Go with a mix of 5-6% SH, 2 cups degreaser (that has sodium hydroxide in it) and some surfactant (go heavy - yeah you will rinse a bit more, but it will help to stick to the wall and do its job). Example - 1/2 gal 10% SH, 2 cups degreaser, 3 oz surfactant and 1/2 gal water.

I find most of the time it takes multiple applications - chem, dwell, rinse. Chem, dwell, rinse. Longer dwell times with misting to keep chem from drying out.

You aint washing vinyl siding here - Just the front in that picture will take you 3-4 hours.

Good Luck!


Wow, 6%?? I have seen a ton of that here in my area, guess I’ll stick to vinyl.

Thank you very much for the info and for the detail explaining! I will do just that and I’ll be updating this post. I’m doing the job in about two or three weeks. Again, thank you for your help! :bowing_man:t4:‍♂

Strong roof mix will work fine. Those look like aluminum clad windows, would not be using sodium hydroxide on. But you may need to hit several times. Charge accordingly.

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Yeah…Sometimes straight up 12.5%.

This kind of stone is very porous. The mold gets under the surface so it is really a chemical game. You are trying to soak the stone long enough for the chem to get deep enough to kill the mold. Doesn’t matter if you use Prosoco ReKlaim or SH. Both are going to take time. SH does have a tendency to bring out an orangish hue so you have to do entire walls, not spot treat so it blends in. I have not had a single client mention it, I noticed it though, maybe they don’t, just glad the ugly is gone.

Be a heck of a before and after! Take lots of pics