Three common sources
Sprinkler water carrying iron from well or older pipework, which leaves fan-shaped orange arcs. Fertiliser containing iron, which leaves scattered spots wherever granules landed and got wet. And metal objects left in contact with damp concrete, which leave a stain in the shape of whatever sat there.
Why pressure does not help
Iron oxide penetrates into the concrete matrix and bonds chemically. It is not a layer on top. Pressure removes what sits on the surface, and this is not sitting on the surface.
What works
An oxalic acid based rust remover, which converts the iron oxide into a soluble compound that can be rinsed away. Application, dwell time and thorough neutralisation all matter — leaving acid residue on concrete causes its own problems later.
The mistake that makes it permanent
Chlorine bleach on a rust stain oxidises the iron further and locks it in. This is the single most common error, because bleach is what people reach for on every other type of stain. On rust it is actively counterproductive.
Tannin looks like rust but is not
Reddish-brown staining under oaks, sycamores and other trees is tannin — organic, not metallic. It responds to an alkaline treatment, effectively the opposite of what rust needs. Treating tannin with rust remover, or rust with a tannin product, wastes time and can set the stain.
Key points
Three common sources
Sprinkler water carrying iron from well or older pipework, which leaves fan-shaped orange arcs. Fertiliser containing iron, which leaves scattered spots wherever granules landed and got wet. And metal objects left in contact with damp concrete, which leave a stain in the shape of whatever
Tannin looks like rust but is not
Reddish-brown staining under oaks, sycamores and other trees is tannin — organic, not metallic. It responds to an alkaline treatment, effectively the opposite of what rust needs. Treating tannin with rust remover, or rust with a tannin product, wastes time and can set the stain.