"Believe it or not I can read a ph scale...
At ph of 3.4 it’s not exactly a strong acid is it now. "……obviously not
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The pH scale is logarithmic and as a result, each whole pH value below 7 is ten times more acidic than the next higher value. For example, pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10) more acidic than pH 6. The same holds true for pH values above 7, each of which is ten times more alkaline (another way to say basic) than the next lower whole value. For example, pH 10 is ten times more alkaline than pH 9 and 100 times (10 times 10) more alkaline than pH 8.
Generally we find (within our trade) rust staining on walls (render, weber, brick work) and patio's ( sandstone, concrete and a variety of lime stones). I would be 5hitting myself useing an acid like Barf f9 on any of these….but as always the Americans have the answer .