Rust on the pad contact area of the discs is the least of your worries if you use the car regularly it just wears off with brake use. The bigger problem is the hubs (think that's the right term) around the centre and in between the thickness of the discs where there vented. Solved this last year after a garage gave me the idea when having either the front or rear discs & pads replaced. They sprayed over areas that don't come into contact with the pads and helps to hold back the rust. I took it a step further when the rust started to return by taking a drill fitted with a wire brush and removed all the rust from around the hubs and edge of the discs, not the area that the pads contact though. Sprayed them up (not break contact area) with very high temperature silver paint and then brushed over some smoothrite and its been doing the trick. That was last Xmas time and is keeping the rust at bay. Previously I had to have the discs replaced because of the rust causing them to fail an MOT so the time taken to do this is extending the life of them I think and keeping one cost at bay.