It's probably brake dust pitting - is it in the corners of the spokes?
Try clay, but it can be hard work. Wonder Wheels will work too, but DON'T leave it on for more than a minute or so, and work it quickly and rinse, or ideally use a MF to rub it on.
The other, pricier option, is Wolf's Brake Duster, which will break it down too, much more safely.