Originally Posted by rlitman
You need an acid cleaner to get rid of the rust stains, but any acid will etch limestone.
I would suggest you find an oxalic acid cleaner (often labeled "wood bleach"). Oxalic acid will be milder on the limestone than other acids, but attacks rust by turning it into a water soluble CLEAR chemical (iron oxalate) that will wash off.
I accidentally sprayed iron (lower dose) on a freshly installed fence. It turned the fence grey in less than a minute. I think I bought some of what you are talking about from the hardware store. It was called "Wood Brightener". It worked great(and fast) for what I used it for. I couldn't begin to tell you if it would work on cement or limestone though...