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try a product on amazon called neutra-sul it is 7% peroxide so a bit more powerful than drugstore grade. I paid about $7 for a gallon. I put the product in a spray bottle and sprayed all of my deck stairs on an overcast day and it completely eradicated the algae. I had to use a deck scrubber to break the algae free but teh parts that I could not scrub dried out and broke free from rain fall. now my 10 year old deck looks brand new and is no longer slippery. Thanks for the tip, I only tried because of this instructable!