Doug,
I have had success with downstreaming oxalic. It won't take much, though you might want to mix it a little stronger than usual. For all other purposes, I use a 4 gal backpack sprayer.
Be careful on stained stamped concrete. We cleaned some rust on a patio and it took that stain off also. We need to restain it with Nu Look now. The homeowner is very understanding and is willing to pay for the materials.