Published on September 13th, 2015 | by Henri


Cleaning or coating limestone

Q.  I have a question about cleaning limestone. We had new stairs installed this past summer and the tops are limestone. For about 3 weeks in August, there are berry trees in the neighborhood that the birds eat, and well, I guess you get the picture. We tried power-washing and used a cleaner designed for the power-washer, but the stains remain. Someone recommended muriatic acid, but I’m hesitant. Do you have any suggestions?

A.  You are right to hesitate before using muriatic acid; it is a very potent chemical that will etch the limestone (i.e., made rough, somewhat eaten up).

A safer way is to make a solution of equal parts Deck & Patio Cleaner and Oxy-Boost from Eco Geeks: Follow directions on the containers.

This mixture is said to be safe and ecological. I have recommended this mixture to contractors and painters who have reported it to be the best cleaner they have ever used.

Another option is to make a poultice with a product such as Comet — as long as it does not contain bleach — and ammonia. Be aware that mixing ammonia and bleach creates a mustard-like gas and can be fatal. Make it into a creamy paste, wet the stones, and apply it to the stained areas with a plastic spatula or similar tool. Cover it with plastic wrap and leave it for 24 hours. Rinse thoroughly.

You should consider sealing the limestone caps with an exterior marble sealer that you should be able to purchase from hardware or box stores. Choose the toughest sealer you can find, such as Stone Tech’s Bullet Proof Sealer. You may also want to try Stone Tech Stone & Tile Cleaner. You can buy both on their Web site, .

However, bird droppings, if that is the problem, are very alkaline and they will eventually cause the deterioration of any sealer except epoxy. You can get clear epoxy, such as the one available from No Skidding at This will give you a permanent seal if you don’t mind a slight sheen.

The product recommended for limestone is Anti Slip Porous Surface (PASS) (Clear) – 8901.

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