Customer had spent the last 5 years washing off his golf cart on his patio. These are battery acid stains, commonly known as "rust" but are not and are actually VERY deeply embedded into the concrete. This is why rust removers are ineffective at removing this kind of stain. Rust removers only handle to topical stain. The stuff we use works on both types of stains- acid burn and rust.
At a minimum, we can get 85-90% of the worst staining out of concrete. Normally we are around 95-100%. This job we got 90% of the worst, 100% of the other stuff. Notice that the white stains are still there. White staining came from concentrated sulfuric battery acid and we have seen it eat through an entire 4" slab of concrete. Not much you can do for the white damage, unfortunately.