No matter how powerful the pressure washer, water alone will not remove rust or battery stains from concrete. There are a number of products on the market that are intended to chemically remove or bleach out the stains. I’m told that Bar Keepers Friend will work, but that it takes a while. I don’t know. Here are my favorite cleaning solutions:
Oxalic acid– Diluted one cup per gallon, oxalic acid will usually remove 70 — 80% of the stain. Any more than 80% is pure magic. Nonetheless, magic happens, and oxalic acid should probably be the first line of defense against rust and battery acid stains.
F9 BARC — This chemical product is mineral in nature, and will remove 90 — 100% of iron and battery acid stains. F9 does not harm plants or grass like oxalic may. So why isn’t F9 the first line of defense? F9 is prohibitively expensive. I use it sparingly to top off a job using oxalic where the acid failed to completely remove the stain. Even then, and in combination with oxalic acid, some iron and battery stains just will not be vanquished.