I used it(hydrochloric acid) with great results on my rust stained bricks and mortar back in 2014.
That was the year I redid all and I mean all the landscaping around our home... we bought this home in 2012 when it was 8 years old. I moved the sprinkler heads further away from the side of the house to avoid the previous owner's mistake.
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It didn't take long and those little bushes filled right in and now need to be maintained.
I used goggles and thick heavy rubber gloves and wore clothes I didn't care about while I scrubbed the brick with a long handled, stiff bristled brush. That stuff ate holes in my blue jeans and long sleeve shirt with every droplet that flicked off the bristles. I came out unscathed but those fumes were pretty bad and had me holding my breath while scrubbing. If I were to do something like this again, I'd get the appropriate canister filter breather unit.
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I widened the landscape beds where necessary and pulled all the rotting woodchips and enough soil to lay broken Spanish Tile about 3 to 4 inches deep over weed block fabric. Seven years later and the landscape beds look almost as good as the day they were rehab'ed and upgraded.