Originally Posted by 97 SL320

Yes, even on a daily driver with a working caliper, the rust ring will grow. Remember, once the surface rusts and gets rough, it sands the pad down faster than the pad wears on a smooth surface preventing it from rubbing the rust off. Once the rust progresses more than 1/2 way, the pad tilts and makes full contact but by that time the rust is so well established it won't wear off.

If the caliper piston on a opposing piston caliper was stuck, the rust would extend the entire width of the rotor surface on the stuck piston side.