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Here in SW Florida, we have well water rust on many house walls and driveways.

I use Muriatic or Sulferic acid at 20% to 33% / with water and a pump up garden sprayer.

It's like 5 bucks a gallon, so it makes 3 to 5 gallons from 1 gallon.

I have used it on most everything-stucco, aluminum, concrete etc.

You have to "learn" to use it, so start out leaving it on for just 30 seconds and then rinse it off with a garden hose or pressure washer.

Here's some pictures. { Oxilic Acid works too, but it costs 20 times as much to use }