Hi John. I have exactly the same problem and have got involved in a few posts on the subject. I think it's terrible that such a "high-end" car should suffer so easily from this sort of thing. My car is covered "" but kept outside, unfortunately. My big mistake used to be washing the car and not taking it for a spin to dry the discs off. The result was the next time I drove the car I would hear a massive knock as the brakes let go of the discs - this was usually after just a couple of days! I now wash the wheels, taking care not to put the hose directly on the discs (it does help a bit!!), and then take the car for short (slow) spin on a side road where I can stop & start without someone rear-ending me! I then have to take the car back home and clean the insides on the wheels again (Carerra five spoke). This time I do it with a damp (not too wet) duster on a plastic frame that I got from the pound shop. For a pound. Then, and only then, can I cover the car and sleep soundly, safe in the knowledge that the four rounds of Edam that Porsche kindly put on my car - instead of some other sort of steel based material "" will not be completely rotten and mouldy by the next time I drive my precious, and I mean precious, car. In fact, the picture you can see on the left is probably the best they've ever looked "" at around 7K miles!
My local independent has quoted me circa £300 per axle "" although it's only the rears that have suffered - as they all seem to. My tax is up at the end of November and I was thinking of declaring the car SORN "˜till next March but I'm a bit worried that if I do the discs will suffer if not used for so long!! (Does anyone know of anything that could be put on to protect them?) Anyway, best of luck. My independent says that the replacement discs will be better than the originals!
Adrian.