Once I power wash all the scum away, is there any way to treat limestone to prevent the mildew from coming back? Or is there something that will clean it up without power washing?

You can try a pool deck sealant. Mix and spray on with a hand pump or backpack sprayer after the stone is dry. I found this slows it down but doesn't stop it.

Our back patio faces north and we have a lot of organic matter from surrounding post oaks and elm. So your position and debris may be playing a role too.

It's been a constant and frustrating battle for us. We are thinking about a backyard remodel. Builder says that removing the light colored stone land stone steps and replacing with concrete preformed stone steps will help a lot. Not sure if I believe him. Need to do some research on that claim because those concrete preformed stones are $.

dr_boogs said:

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You can try a pool deck sealant. Mix and spray on with a hand pump or backpack sprayer after the stone is dry. I found this slows it down but doesn't stop it.

Our back patio faces north and we have a lot of organic matter from surrounding post oaks and elm. So your position and debris may be playing a role too.

It's been a constant and frustrating battle for us. We are thinking about a backyard remodel. Builder says that removing the light colored stone land stone steps and replacing with concrete preformed stone steps will help a lot. Not sure if I believe him. Need to do some research on that claim because those concrete preformed stones are $.

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Limestone leaches minerals that the mildew eats. Concrete's leaches it too, but at 1/1000th the rate. It will slow it down but not stop outright. Is washing it once a year vs. once a month worth it to you?

Absolutely.

Take a look at Chitzen Itza or any Mayan or Aztec buildings. If you thought limestone stays white you were dreaming. Sorry.

Here's what I do with my pool and cabana stone every spring. Spray bleach diluted to 50%, power wash gently to not etch anymore of the stone than necessary, let it dry completely, rebleach. About once per month after that spray with bleach. The only time I'll have to power wash again is if it's rainy for a long time and my steps get discolored from the mud. This works for me.