Have you got a stone path or patio that looks like this?
Did you use grass fertilizer recently?
What you are looking at is Rust. Many fertilizers on the market today contain a small percentage of iron. Fertilizers with iron are commonly used to create a greener, healthier lawn by preventing and correcting yellowing from iron or manganese deficiencies. But they also have a downside. If you manage to get any fertilizer particles on your stone path or patio and don’t remove them, the iron in the granules that settled onto your nice patio or pathway, probably by accident whilst you were spreading the fertiliser, when wet will oxidize and shortly afterwards start to show up as rust stains. These rust stains are extremely hard to remove until now.
It was originally formulated for textile use, getting rust stains from clothing, however people discovered it also works at removing rust spots from concrete paths and driveways, it worked so well in fact, that HG included it on the product label.
All you have to do is soak a cloth in the product, place the cloth on the rust spots for a few minutes to give the liquid time to work into the concrete, leave it to work for a couple of hours.
Get a bucket with hot soapy water and a brush and scrub the wet brush over the stains and they will just lift right off, leaving you with a pristine looking path or drive again.
Unfortunately the product is only available in one size (50ml) but what price can you put on something that actually works even though it was never originally designed to do so.
Of the course the best answer is to the read the small print on the container of Grass Fertilizer before using and take heed. Avoid getting it on your path or patios in the first place!