We have some retaining walls in our back yard made with Austin stone. One of them has a thin layer of mold or mildew growing on it that I can't quite scrub off. Any suggestions for getting rid of it? Should I just spray bleach on it?

bleach and high pressure water

Mix the bleach with water 1 part bleach to 4 parts water to start ,more concentrated if you need to. Spray on with high pressure bottle and then rinse off within a few minutes.
If you get it on your paint and it stays on too long, bye bye paint.
Wear clothes you don't want anymore

In the process of doing this at our house. Chlorine bleach will hurt the landscaping and high pressure water may affect the surface of the stone. Mix oxygen bleach powder with water per the instructions for outdoor use, scrub it on with a nylon brush (not the really stiff 'deck-scrubbers', just a regular nylon scrub brush). Let it sit for a few minutes, spray it off with a sprayer nozzle attached to a garden hose. Repeat as needed. Shouldn't hurt the plants as it rinses off and shouldn't erode the surface of the (really pricey!) stone.

I've been the treasurer of a historical cemetery association for 15 years, so cleaning stone is not a new subject. The following information is from Tracy C. Walther-Architectural Conservator.
Granted, extra care is used when historical gravestones or monuments are concerned, but if you want to do the least damage it may be of use. Also, avoid using a high-pressure sprayer and go with a soft-nylon bristle brush if needed.

quote:

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C. Alkaline, Corrosive, and Biocidal Cleaning Materials

1. Sodium Hydroxide (e.g., "Borax", Sodium Hypochlorite (e.g., "Clorox" "liquid chlorine" is not recommended for general cleaning of stone.

2. Calcium Hypochlorite (e.g., Chlorine, "HTH," "Shock Treatment": Calcium hypochlorite or chlorine is effective for the removal of biological growth. It is a granular product that is not to be confused with "liquid chlorine" or sodium hypochlorite. Calcium hypochlorite is available from swimming pool suppliers. A suggested cleaning solution is one ounce calcium hypochlorite to one gallon hot water. This product should be used only when a waterhose with a good water pressure (e.g., 55 psi) is available.

3. Ammonium Hydroxide (e.g., household ammonia): Solutions of household ammonia are recommended for cleaning light colored stones. Ammonia is particularly effective for the removal of biological growth. One cup ammonia to one gallon water.

4. Quatemary Ammoniums (e.g., algaecides or biocides for swimming pools): Quaternary Ammoniums have a slightly different chemical structure than ammonium hydroxide. They are especially effective for the removal of biological growth, particularly stubborn black algae. Quaternary ammoniums are available from swimming pool suppliers and list ingredients such as alkylbenzyl trimethyl ammonium, benzyl alkyl dimethyl ammonium chlorides, or benzyl aklyl dimethyl ammonium bromides.

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(Edit: Ignore the winky faces following trademarks, nothing is visible in the text that I copied and pasted from the website.)

[This message has been edited by ComfortAg (edited 6/29/2007 10:00a).]