For the past 6 years I've been looking at my very grotty front garden wall and wondering what to do about it. Seems that it was built 30 years and has never been touched. I've seen them painted; it just does not seem to work very well.
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Use a wire brush, then 500g of caustic soda in 6ltrs of cold water. Apply to the wall with a hand brush. Wear gloves and don't get this stuff on your skin or in the eyes. Wait for about one hour, then jet wash off.
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I used masking tape around the joints on the top stones, then a narrow trowel for the sand and cement.
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It's the best part of a days work but worth it.