Originally Posted by Outlaander

The one and only reason the cast-iron rotors' surface looks really good is that the pad wears off any film of rust that does indeed form when the vehicle is at rest.

But there is almost always a very small portion of the outer edge of the rotor that the wearing part of the pad does not touch, so the rust just keeps building up and forms that typical ring of rust.

Even though the ring is always there to at least some extent, there usually is no noise when braking. The rust can and will never encroach under the pad because, again, the edge of the pad just brushes it away, forming a nice razor-sharp border between the shiny rotor and the rusty ring.

But when the wearing part of the pad wears down to very thin, now the metal "backing" of the pad, which can be just a bit larger than the wearing part of the pad it supports, can begin to rub on the rust, causing a light rubbing/grinding, metal-on-metal noise. When the pad wears out completely, the entire metal backing begins to touch the rotor and then the noise gets much louder and starts cutting grooves in the rotor.

If you replace your present OEM pads with Mitsubishi pads, again, there is a very good chance that the grinding sound will disappear completely, because the shape of the wearing-part of the new pads will be identical to that of the old.

If you replace with aftermarket pads, they may be a slightly different shape than the OEM and the wearing part of the pad may overlap the rusty ring just a bit. In that case you may hear some slight grinding. At first the rusty ring will very quickly cut a corresponding groove in the pad that will be almost as deep as the height of the rust above the rotor. As the groove is cut, the rest of the pad will very soon make contact with the smooth part of the rotor and the groove will not be cut deeper. From that point on, because rusted cast iron is not anywhere near as resistant to abrasion as un-rusted cast iron is, as the pad wears down naturally on the smooth rotor, the portion of the pad over the rusty ring will wear down the rusty ring the moment the rest of the pad allows it.

So, if the rusty ring stands say 1/16" above the shiny part of the ring when the old pads are removed, after the new pads are installed, the first application of the brake will force the pad to wear down the rust a considerable amount as the rust cuts the groove in the pad. Because of the "work" the rusty ring has done on the pad, now the "top" of the rusty ring sits well under the 1/16" above the shiny part of the rotor that it used to be. Once the wearing part of the pad over the shiny part of the rotor decreases in overall thickness that amount, the rusted portion of the ring will have been worn down by the pad to shiny metal. That now-shiny ring will still be slightly higher than the rest of the rotor, but only an insignificant amount and, because it will now be as shiny as the rest of the rotor's surface, there will be no brake noise -- noise that will likely have stopped very shortly after the new pads were installed.