Cleaning flagstone can take time and you will be using different types of chemicals. My dad used products from Prosoco because they are such high quality.
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He has retired and hasn't used the latest cleaners, so I'm going to search around the internet and ask professionals about quality products. I'll report back to you with my free newsletter, so make sure to sign up for it.
Most often stone turns black because stone is very porous and absorbs all dirt and dust from the outside world.
When you have had enough of your nice looking stone turning black, the solution we use is ReKlaim cleaner from Prosoco.
There is a three part process to it. You need to mix the ReKlaim cleaner and activator together. You apply that to the surface, but you SHOULD ALWAYS APPLY IT TO A SMALL PART SO YOU KNOW WHAT THE RESULTS WILL LOOK LIKE.
After you are done with that, you will follow up with a Limestone and Masonry Afterwash.
The total instructions are at Prosoco. And feel free to call them. They have some of the friendliest customer service representatives that I've ever talked to. They can help with any masonry stain problems you might have.
Their number is 1-800-255-4255.
If you have mold or green on your stone, you can use Bio Wash. It is a much milder solution and it is easier to apply. That is also on their website.
Good luck with your cleaning!!
I will have more info about keeping your stone clean in my free Newsletter. Just put in your first name and e-mail. All the best, David