Where i live 25% or more rust on rotors gives a instant vehicle inspection failure, a lot of pads and rotors with a lot of thickness still left will be replaced due to this. Main reason for premature rotor replacement is salt/rust corrosion which are a factor in places where the roads are salted during winter/snow season. I work on vehicles so i see this issue often. Salt and grime gets between rotor and pads, grinds along and makes a uneven contact surface. Same thing happens when salt and grime gets between pads and caliper, pads get stuck and there is uneven contact surface.
Best measure to combat this is try to wash brakes so salt don`t settle and brake hard few times. But i also see that economy cars never have calipers treated with any paint or coat, they are just a bare metal.
Best measure to combat this is try to wash brakes so salt don`t settle and brake hard few times. But i also see that economy cars never have calipers treated with any paint or coat, they are just a bare metal.