What is the best way to remove rust stains from concrete?
Mix one part of sodium citrate in 6 parts of water, then add 6 parts of commercial glycerin. Make a paste by mixing this liquid with powdered whiting and spread it on the stain in a thick coat. Allow to remain until dry, and then either replace with fresh paste or moisten with the remaining liquid. Stains that are deep and old will require several such treatments, and it may take a week or two before the discoloration is completely removed.