I got a request today to remove fertilizer rust stains on a driveway and sidewalk. Scott's is responsible for the issue and will pay for the damage that they did. Likely the granules got sprayed on the concrete and the sprinklers got them wet and caused rust-like stains that look like little comets. I tried this on a sidewalk a year ago using a strong oxalic acid mix with no success. Which acid or specialty chemical will get this off and what is the mix ratio to safely do it? Thanks for any help!