Today and yesterday I've been taking a look at my brakes. It's a bit weird because the car is stopping absolutely fine and I can't feel any brake-related troubles, but my rear brakes look like this:
This all started over the Xmas period when I wasn't using my car. Discs looked absolutely fine up until I took some time off work and then I just didn't drive my car for a while. Over a period of about 4 weeks the most I did was a single 30 mile trip. When I had to do a drive to Ireland in mid January I checked over the car and saw the brakes were like that. Obviously a little bit of surface corrosion is no big deal and it cleans itself up, but the rust on the outer edge of the discs will not clear up no matter what, even if I do some heavy braking. Those pics are after having driven the car nearly 2k miles since I first saw the problem.
Perhaps more concerning is that I only put those discs and pads on the car back in August so they're far from worn. I did clean the car a few days in to my time off and then left it so that's probably what caused it, or at least significantly contributed to it, along with me living near the cost. So yesterday I took the rear calipers off, cleaned everything up, lightly sanded the brake carriers and the pads down and put it all back together.
You can clearly see on the pad where the areas contacting the rusted patches of disc are. Today I did the same to the front brakes as I noticed one of the front discs also has some rust buildup that won't clear. Hopefully now I've done that, a bit of harsh braking might just clear it up. Just need to go out for a drive when it's quiet to try it out. It is strange though, I can't figure out why it wouldn't just clear up on its own. I guess the rust was just that severe and it compromised the pad material somehow.