Had this issue on C4S except the entire rotor hat/hub was rusted on the right rear. Also had rust dots in a different shade all over it. Looked diseased.

I used a strong wheel cleaner/degreaser.. Soaked it up real nice, scrubbed away with various brushes and steel wool then pressure washed it off. It took it all off and it only very slightly returns.. I can live with that.

 

The larger and heavier front rotors are two-piece with aluminum hat sections to save weight. The smaller rears are one piece cast-iron.

 

As others have said remove the surface rust and use a MB to dry the car, I just use a Toro electric leaf blower, their best one, it works great. I blow dry the entire car and wheels after washing, really should eliminate fine scratches. Love that SB color!!

 

Common on German cars. They don't paint the rotor hats very well (super light coat) and the hats start to rust over time. Porsche seems to be better than BMW though.

Anyway, it's easy enough to paint them with a high-temp paint (obviously need to remove the rust before painting). I opted to do a complete swap on the rotors even though mine still had decent life in them. Bought the car CPO w/ 15k miles on it and I am OCD about these things. To me, it was easier to just do a complete brake job and pre-paint the rotors before mounting as opposed to trying to remove all the rust on the hats and then paint them (also leaving the car out of commission for days while the paint dried/cured properly).

Here's mine:

 

 

^nice!

 

Edited my post after I realized this was a GTS and not a GT3 rotor. Anyway, it's not warranty item in any case because the rust is from something else (rotor surface, most likely), so the rotor hat is pretty much just dirty. Clean it, maybe paint it with some heat-resistant paint for better looks. Porsche should just offer two-piece rotors on GTS and S as well - single-piece rear is too low-grade compared to the rest of the car.

 

If you don't want to go to the trouble of painting.

You can get a resonable repair by wire brushing off the loose rust from the rotor hat and then using a craft product called Rub 'n Buff. Pewter was a good match on my VW touraeg and mini cooper. Silver leaf may work too. Costs about $7. Takes about 5 minutes per wheel. You wipe it over the rust with your finger. No masking needed!

It holds up well if you don't daily drive in winter salt. Otherwise it needs to be renewed when ever you change/ rotate tires.

I'll add some before and after pictures if you are interested.

It's kind of my "secret" so don't tell anyone. 😉

 

I never heard of this stuff, but looking at their website it looks promising! Thanks for sharing your secret. I have found using a scruffy pad with Simple Green takes off some of the rust....

 

before and after pics would be great

Rub and Buff holds up well to the heat?

Thanks

 

It seems like it holds up well to heat. I learned about it on a BMW airhead motorcycle (R100) site where folks use it to restore engine cases.

I'm swapping snow tires soon, so pictures to come.

 

Here are some pics of the rear of my daily driver Touareg. 60000 miles, in WI and IL salt. I used a toothbrush to apply instead of my finger. It's like antisieze, it gets everywhere.

Before:


After wire brushing, wipe with lacquer thinner, apply rub 'n buff with a soft toothbrush. Clean rotor of any paint.


Done in 5 minutes or less. You might have to rotate wheel to get under the caliper if you need it perfect. I can usually reach thru wheel for any of those spots.

 

I wonder if silver antisieze would work in a pinch?

So I'm testing antisieze on one side of our Mini Cooper to compare over
​​a few months. It seems thinner than rub n' buff, and I'm not sure it will dry correctly. It's easy to remove both with lacquer thinner.

Update: antiseize doesn't work! Dead end.

 

That looks pretty darn good. Thanks for posting pics.