Yes. For stone like in the picture (limestone), you want flow over psi. 4 gpm is a good amount. Dial down the pressure to 2-3k. The higher the psi, you actually drive stains/contaminants further into the stone.
This building is similar to the one that you are bidding on. It’s made of brick and cast stone (a concrete masonry) product. It is 2 stories on three sides and almost 40,000 sq.ft. We were able to clean it in 2 1/2 days. 2 days cleaning the first floor using ladders and 1/2 day cleaning the 2nd floor from a lift. That’s with 3 men pressure washing and a spotter when we were on the lift, all using 4 gpm machines. We used SH on the brick and cast stone, applied with a 12 volt system and a pump sprayer and rinsed with medium pressure. We charged 10K.
I find on the pre-cast stuff, a long dwell time with repeat applications is necessary to get the SH to soak in to where the mold is to kill it. Do you find the same thing?
My mind goes to making a gel like substance with something like roofsnot and SH that I would brush on to see if that would penetrate more quickly or at least eliminate the reapplication every 5 minutes so I can work on other stuff.
What percentage SH are you applying and are you downstreaming it or using a direct application?
Give me an hour - gonna go drop-off trailer and i’ll swing by to take pictures of various sections -
Current project has 1400 ft section with a decorative pre-cast divider on wall as well as caps and also some split block pillars with pre-cast pyramid caps on top.
OK - Start with this picture -
This is a prime example of what I run into. This particular cap looks amazing (Pillar is clean. I think it is still wet for the cleaning this morning.)
Now look at this picture -
This is a pillar next to the first. Same exact process but you can still see the mold inside the stone.
I use direct application starting with 4-5% hitting. If after several applications it just aint getting there, then continue to up the SH%. I usually encounter these on larger jobs and I just keep hitting them over a few days and they eventually come out like the first picture.
I see the same thing with honed (not polished) stones as well. You just have to keep hitting it until the SH gets soaked in enough to kill the mold. I find if I keep going back every 10 minutes and reapplying and keeping the surface wet, it seems to work better or quicker at least but not always.
I have experimented with pre-wetting the stones and apply dry with not much of a difference in results between the two methods. I think pre-wetting edges out dry application just a bit. I up my surfactant as well to make sure the chem sits still. I think I went too far this morning as coming out of the pump up it was like syrup!
Just did the math - I think this year I have already done close to 200,000 sq ft of stone restoration. I need some simple vinyl houses to wash for a week or two
Yes, it does take multiple applications. I applied a 50/50 mix or so, on this cast stone multiple times but it didn’t budge until we hit it with a little pressure.