The corrosion does no harm, the brakes work as well if not better as it gives a similar effect to grooving. When you get the brakes they come with a waxy preservative and you have to remove it with brake cleaner. I would not want to do this with them on the car as you might contaminate the pads and the benifit is limited as they still corrode under the pads. The front discs on the MG sit from October to April the car is just rolled around a bit to stop the tyres flatting, and thats it, they may squeak a bit first time you use them but from then on fine for another year. I have had a car fail MoT where the outside rim has corroded away, but even here if its not too far into the swept area the trester can still pass you at their discretion.
