You washed the car on Saturday. By Wednesday the wheels look like you did not, and the rest of the car still looks fine. That is the part that annoys people, and it makes it feel like the wheels were skipped.

They were not. What is on them is not road dirt. Every time you brake, the pad is ground against the disc and the disc gives up hot iron particles that land on the wheel and embed themselves in the finish while they are still hot. That is why soap does not shift it and why a sponge just moves it around. You are not washing off a layer, you are trying to wash out something that has bonded into the surface.

The colour change is the whole point of a product like this. It contains a chemical that reacts with iron, and the reaction runs purple. So the wheel bleeding purple is not a gimmick, it is the visible proof that the bonded particles are being converted into something that will rinse. When the running stops changing colour, the iron is off.

That changes the job from scrubbing to waiting. Spray, let it work, rinse. The instinct to attack it with a brush halfway through is what wears the lacquer on a wheel over years, and it is unnecessary here.

Here is the limit. It is pH balanced, which means it is safe on finishes and also that it is not the strongest thing you can buy. On a wheel that has never been decontaminated and has years of build-up, one pass will visibly improve it and will not finish it. Plan on two.

It also does not touch tar or brake grease. Those are hydrocarbon, not iron, and no amount of this will move the black speckle along the bottom edge of a wheel arch.

And it does not stop the dust arriving. Some pads shed far more than others, and if yours are the heavy kind you are buying a faster clean, not fewer of them.