Rust

Iron oxide, which forms wherever a pan is left wet or stored in damp air. It starts in pitting, at the rim, and under the handle, and it is removable as long as the iron underneath is still sound.

Forty minutes of a watched vinegar soak plus steel wool took a 0.4 mm rust layer back to grey iron; three baked layers of grapeseed oil at 230 °C rebuilt the pan.

Oiling over rust does not fix it. The oil seals the rust in, the rust keeps growing under it, and the layer lifts within a week. Rust has to come off first, in full.