My concrete slab got rust stains on it because the lawn sprinklers were fed from a well that was full of iron. It took about eight years for it to turn a deep rusty color. So I worked out a system that was cheap, easy and effective although one of the steps did take a bit of elbow grease. The whole process only took an hour or so for an 8-ft. x 20-ft. slab. You’ll need to buy a gallon or two of for about $15, a pump up sprayer and a good stiff brush that you can screw into a sturdy handle.
The first thing I did was power wash the slab to get rid of any surface dirt. I’m not sure power washing is important, but I figured any surface dirt might insulate the rust deposits preventing them from being dissolved by the next step-the acid.