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Rust is not a layer on the pan. It is twice the volume of the iron it replaced — so scrubbing it off only rewinds the clock.

Orange powder that comes off on a cloth · Pale patches after one night with water in it · Dark speckles that will not scrub out · Flakes of black film peeling away · A metallic smell · Food sticks in the same ring every time · Grey spots where the film has gone · Water beads in some places and spreads in others
FreeHot water, a stiff brush, dry it on the flame, one thin coat of oil
$5A sheet of 320 grit, to take a pit back to bare iron
$25–60A new pan — cast iron for searing, carbon steel for tossing
Which kind of rust? What seasoning actually is
Easy · 30 min · Then cook in it

The pan in section — the polymerised film, the bare iron under it, surface rust, pits eaten into the metal, and what seasoning builds up to

diagnose
What failed
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Where
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State
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what to touch fault correct Click the wall, a pit, or the film

Seasoning is polymerised oil, not a coating you put on

When an oil is heated past its smoke point, its double bonds oxidise and the molecules cross-link into a solid network. That is the same chemistry as paint drying, and it is what makes a pan black and slick. Two consequences follow, and both of them explain failures you have probably had. The first: raw oil is not seasoning. Wiping a pan with oil and putting it away leaves a film that water lifts off and that goes rancid — a layer that protects nothing and smells. The second: thin coats beat thick ones. A thick layer of oil cannot all reach the same temperature, so the outside sets while the inside stays soft, and the whole sheet lifts off in flakes within a month. Three thin layers, each heated until it smokes and left to cool, give a coating that grips.

Rust works the other way round. Iron oxide takes up about twice the volume of the iron it came from, so it cannot sit flat in the place the metal used to be: it rises, becomes a loose, crumbly crust, and holds water against the fresh metal underneath. That is the mechanism behind a pan rusting through. Which means scrubbing the rust off is not a repair, it is a rewind — and unless you seal the bare iron in the same session, the rust you removed has already started again by evening.

Three places, three different answers

Orange powder on the surface, which a fingernail scrapes away to reveal dark film underneath, is the cheap one. Hot water and a stiff brush — no detergent needed, though it does no harm either — then dry it on the flame rather than with a cloth, until the last of the water has flashed off. Then a thin smear of oil, heated until it smokes, and the pan is sealed. The whole job is five minutes, and doing it in the hour after the rust appeared is the difference between a patch and a scab.

Dark pitted speckles, which will not scrape off and feel like dips under your nail, mean the rust has eaten into the metal. Scratch at one with a needle and see whether it bottoms out in a pit. A pit has to be ground back to bright iron — 320 grit, circular strokes, because here you are flattening rather than following a grain — then washed, dried on the flame, and reseasoned from bare metal. It will look pale and ugly for a few uses. It comes back.

Film peeling in flakes means the coating itself has failed, which almost always traces back to water standing in the pan or a night in the sink. It has to come all the way off, not just where it looks loose: hot water and bicarbonate to soften it, a scraper, then 120 grit until the whole interior is bare grey iron. Then three thin coats, each heated to smoking and cooled before the next. Doing it in one thick coat is how people end up back here in three weeks.

Three things that make it worse. Do not attack it with a steel wool pad in straight lines. It takes the rust off and takes sound film with it, and the scratches it leaves are the low points where water will sit next — so the rust comes back along the scratches. Do not soak it to soften the rust. Soaking is rusting: water reaches the iron under the film and lifts it off from below. And do not wipe oil on cold and call it seasoned. Oil that has never reached its smoke point is not a coating, it is food for the next problem. Beyond technique, know when to stop: a hole or a weeping spot, a loose rivet at the handle, or a base that has bellied so the pan rocks on a flat hob — those four are a new pan, because a wobbling base heats unevenly and no amount of seasoning fixes geometry. And one thing worth unlearning: washing-up liquid is not what kills seasoning. A fully polymerised film is a cross-linked solid and detergent cannot dissolve it. Overnight soaks and the dishwasher are what actually do it.

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