This would be my first vertical job. I am scheduled to do it on Tuesday the 4th. I would appreciate any and all tips and suggestions.
Remember I am using a 4 gallon fumigator to deliver my chemicals. Do plan to take some plastics to cover the plants so I don’t get much chemicals on them.
For chemicals. I get 12 to 13 percent SH. I am thinking of using two different batches. The strong batch doing one part water one part SH to bring it down to about 6 percent, then add some liquid detergent and some coconut scent. The strong batch would be mostly for the top, and for where it looks black.
Then do a milder mix, going two parts water and one part SH which should bring me to about 4 percent sh. And use that for the rest.
That said, the more I think about it, not sure if that would work. I only have one backpack, and I’ve read on walls/verticals I should always start from bottom to top because I could create bleach stains otherwise. Does that mean I should use the strong mix in the whole thing?
This might be out of your depth, lots of opportunity to damage property and yourself especially. Have you considered a respirator, PPE, neutralizer, oxalic etc?
I do have oxalic acid, should I use that instead? “This might be out of your depth”, at the end of the day, it is not brain surgery. Yes, I could damage property, mostly stain it, that is why I am asking for advice on this forum to perhaps learn from mistakes others have made without me having to make them.
Lets say you lived here in Cancun and that was your entrance. You wouldn’t have many options as to who to hire to clean it for you. Part of the reason I decided on this venture was because there seems to be very very little competition out there.
After MUCH MUCH research we were able to touch base with a guy that does pressure cleaning for resorts. He seems to be the most profesional guy we were able to find, and he works with similar equipment than I do. He doesn’t use a downstream injector either. In total we’ve only been able to find about 5 or 6 other people offering pressure washing as a service, one of them uses an electric pressure washer with only the wand to do all his jobs including concrete driveways. Another guy says he only uses chemicals on “problem spots”, so basically no full pre or post treatment.
Basically I want to become the “go to guy” for pressure washing. Instead of “guessing” or learning by trial and error figured I would get advice from real professionals.
According to ChatGPT, these are the steps I should take:
Step 1: Preparation
Assess the Wall Condition – Check for cracks, loose stones, or mortar damage. Fix any weak spots before pressure washing. Protect Nearby Plants & Surfaces – Cover nearby plants with plastic sheeting to prevent chemical damage. Pre-Wet the Surface – Use a garden hose to saturate the stone to prevent excessive chemical absorption.
Step 2: Apply a Cleaning Solution
Choose a Cleaner
For Algae, Mold & Mildew: Use a mix of Sodium Hypochlorite (SH) and water (typically 3:1 water-to-SH ratio).
For Heavy Stains: Use a commercial stone cleaner or mix 1 cup of Oxiclean per gallon of water.
For Organic Stains & Rust: Oxalic acid works well.
Application Method
Use a pump sprayer or downstream injector on your pressure washer to apply the solution.
Let it dwell for 5-10 minutes (don’t let it dry).
Step 3: Pressure Washing
Choose the Right Pressure & Nozzle
Use low to medium pressure (1500–2500 PSI) to avoid damage.
Use a 25° or 40° nozzle (green or white) for even coverage.
If deep cleaning, use a rotary surface cleaner for uniform results.
Washing Technique
Start from top to bottom to prevent streaks.
Keep the nozzle 12-18 inches away from the surface.
Use long, even strokes to prevent etching.
Step 4: Neutralization (If SH Was Used)
Apply Oxalic Acid (for brightening stones) or Sodium Metabisulfite (to neutralize SH residue). Let sit for 5-10 minutes, then rinse thoroughly.
Step 5: Final Rinse & Dry
Rinse with low-pressure clean water to remove all residues. Allow the wall to dry completely before sealing (if desired).
Optional Step: Sealing (For Long-Term Protection)
If you want to prevent future mold, algae, and water damage, apply a breathable stone sealer once the wall is fully dry.
Pro Tip: Test the pressure on a small, hidden area before washing the entire wall to avoid damage.
Based on that, could someone give me more detailed tips?
Just hit the whole wall with 6%, then green tip from a distance and hit the wall. Don’t use oxalic acid on the wall. If you mix it with chlorine you’ll have mustard gas. You could probably use the green tip first then hit everything after with the chlorine and it would come out the same. Just pretreating would make it come off a little easier.
I didn’t go down the rabbit hole researching, but found it interesting that oxalic is used to “neutralize” SH, since I’ve had read it could be dangerous to mix them both.
So I did a quick google search and found:
No, you should not attempt to neutralize sodium hypochlorite (bleach) with oxalic acid; mixing these two chemicals can be very dangerous and potentially create harmful fumes due to their strong reaction with each other, which could even lead to an explosion in concentrated forms.
Instead, to neutralize sodium hypochlorite, use a mild reducing agent like sodium thiosulfate or sodium bisulfite which are readily available and safer options for this purpose.
It seems the ChatGPT instructions are a bit misleading. After the SH is rinsed off I could use Oxalic acid to brighten up the stones, but it wouldn’t be neutralizing the SH. So I guess if I decide to use Oxalic after SH I should rinse the SH very well and let it dry before I apply Oxalic acid.
Sheesh, first in a hidden spot if possible, hit the painted block, if that is what it is, with your strongest straight SH in a pump up. Let sit for 10 min, rinse and make sure it doesn’t hurt the paint. That top part you’re probably going to need to hit with straight SH several times, about 5-10min in between. I would wet wall, spray the top part with your SH until about 95% of the stain gone then hit with medium pressure, about 800-1000psi. Keep wall kind of wet with water. The painted part, can probably go a good bit less strength on the SH, except for the really bad parts. Just a little surfactant to help SH penetrate.
Don’t use ChatGPT for this type of info, it searches the internet for its answers, the internet is full of wrong answers.
You’re on the right track by coming here and asking, especially as you’re just starting out. Many of the experienced members with a wealth of knowledge are more than willing to help… you just need to show that you’ve made an effort to search before asking (which from all of your posts so far, it appears you have been doing).
Thanks to everyone that gave me suggestions. In about an hour I will go do the job.
Some suggested as low as two percent on the SH, others as high as six. My plan of attack is to use four percent, going with a three to one ratio of water and 12 percent SH mix. Wet, put on chemicals, wait 15 minutes making sure it doesn’t dry out, rinse, and repeat on the areas where it didn;t fully clean.
I plan to do it in sections since I will be doing it in the middle of the day so sun will make it hard to keep it wet for 10 to 15 minutes.
Once I have it all cleaned out, plan to rinse everything very well, then test a small portion of the white stones with some oxalic acid mix. If I see positive results plan to hit the white stones with oxalic, if I don’t see much difference I would skip that step.
Will post before and after pictures once I come back.
You get different suggestions on strength because it’s a combination of % and dwell time. You could clean that with 1% or 12.5%, just depends on dwell time and number of coats. I personally prefer to go less on the strength and more time to reduce risk. I plan jobs in my mine well before I start and areas that need the most time is where I start and finish with the rest of the work in between.
I think your 4% plan is plenty strong enough for this job. Hit it a couple times, take your time, watch what you’re spraying SH on, keep things wet and it will come out great.
Also, maybe just me, but I wouldn’t even bother with Ox. Just rinse everything well.
Well actually that is not the finished result as I did end up doing the whole front sidewalk also. Turns out the other side, the inside, was actually worse than the front as it gets much more shade.
Assess the Wall Condition – Check for cracks, loose stones, or mortar damage. Fix any weak spots before pressure washing.
Choose a Cleaner
Application Method
Choose the Right Pressure & Nozzle
Apply Oxalic Acid (for brightening stones) or Sodium Metabisulfite (to neutralize SH residue).
Rinse with low-pressure clean water to remove all residues.
Allow the wall to dry completely before sealing (if desired).
Pro Tip: Test the pressure on a small, hidden area before washing the entire wall to avoid damage.
