Question
Should we acid wash our classic oak flagstone patio and pool decking?  Will this bring out more rich color or damage the stone?

Answer
Joyce:

The answer depends on what you are attempting to remove.
If it is just old dust, dirt, mildew, mold etc. then a mild bleach and light pressure wash should do the trick. I don't know that acid will do anything but pit the surface and burn your nose.

The only reason to use acid would typically be to prepare a surface for mortar or concrete to be applied. We use acid on old slabs of concrete when we plan to put down new stones or a thin layer of stamped concrete.

So I guess I am saying yes acid will typically roughen or damage the surface (although you won't notice it without a magnifying glass).

Additionally, acid will not bring out the color unless it is removing something that is obscuring the color. For example if the contractor had left a haze of mortar wiped across the rock, then the acid would definitely help in removing the haze.

To bring out the color of a stone consider sealing it with masonry type sealant. I use a product with the trade name 'Rez-Seal'. In my experience, the best sealants have a high solids count and are oil based. Also I go with a matte finish unless you want a very slick waxed look.
Sealants will bring colors to life but some people do not like the shinny look they typically give so I always recommend a test area be done first.

Also, if you seal your stone it can make the stone slippery if the stone is fairly smooth to start with. You may need to add a non-slip additive like "Shark's Skin" or a silica sand into the wet sealant. Also sealing the stone closes many of the small pores in the stone and will end up making this stone much hotter to your feet. Many find that well sealed stone out side in full sun is too hot to stand on in bare feet.

Finally, sealing the stone will make it last longer. Sealing it means you block water from entering the cracks and pores of the stone and lessen the chance of water penetrating and then freezing and breaking the stone loose. It also makes it harder for moisture to stay in the stone which supports mold and mildew.

Sean J Murphy, LA,ISA, LEED AP
Please check out my blogs and articles on landscape topics like this on my websites.
http://www.seanjmurphy.com
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