TL;DR
Cast iron gets cleaned while it is still warm, dried on the burner, and oiled before it goes back in the cabinet. Every step exists to keep water off bare iron, because rust forms in hours. A quick squirt of dish soap is fine, but a long soak in the sink is what actually destroys a pan.
What seasoning is, and what damages it
Seasoning is not a coating that was sprayed on. It is polymerized oil -- fat that has been heated past its smoke point until it bonds to the iron as a hard, slick layer.
That layer does two jobs. It releases food and it keeps moisture off bare metal. When people say a pan is "ruined," they almost always mean the seasoning is gone and the iron underneath is now rusting.
Three things strip seasoning: long soaking, aggressive detergent contact over time, and prolonged simmering of acidic foods like tomato sauce or wine. A short wash with dish soap does not, despite the old advice. Modern dish soaps are not the lye-based products that gave the rule its start.
Source: cast iron cookware manufacturer care guidance
What is the right way to clean a cast iron skillet?
Clean it while it is still warm. Rinse with hot water, scrub with a chain mail scrubber or a stiff brush, add a drop of dish soap only if there is real grease, rinse again, and dry it immediately with a towel. Then set it on a burner for a minute and rub in a thin coat of oil.
The four-step routine, in order
Rinse warm. A warm pan releases food far more easily than a cold one. Do not shock a screaming-hot pan with cold water, though, since thermal shock can warp or crack the iron. Let it come down to comfortably warm first.
Scrub. Chain mail is the single best tool for the job, because it removes stuck food without touching the seasoning underneath. A stiff nylon brush or a plastic scraper works too. Skip steel wool and scouring powder unless you are deliberately stripping the pan back to bare metal.
Dry completely. Towel it, then put it on a burner over medium heat for one to two minutes until every trace of moisture is gone, including in the handle joint and around the pour spouts. Never let cast iron air dry.
Oil. With the pan still warm, add about half a teaspoon of a neutral cooking oil and spread it over the whole interior with a paper towel, then wipe it back off so the surface looks barely damp rather than wet. Excess oil turns gummy and sticky in storage.
Stuck food that will not come off
Fill the still-warm pan with about half an inch of water and bring it to a simmer for a few minutes. The steam lifts stuck-on food, and you can scrape it free with a wooden or plastic spatula. Pour it out, wipe, and continue the normal routine.
For a truly welded-on layer, pour in a tablespoon of coarse salt and scrub with a folded paper towel or a cut potato. The salt scours without cutting the seasoning the way a metal pad would.
Do not reach for the baking soda and peroxide approach that works on burnt sheet pans. Long soaks and strong alkalis are exactly what cast iron cannot take.
How do I fix a rusty cast iron pan?
Scrub the rust off with steel wool or coarse salt until you see gray metal, wash with soap and water, and dry immediately on a burner. Then re-season: rub on a very thin coat of neutral oil, wipe it nearly dry, and bake upside down at 450 to 500 degrees F for an hour.
Re-seasoning, in a little more detail
Put a sheet of foil or a baking sheet on the rack below to catch drips. Bake the pan upside down so oil does not pool.
The key word is thin. A thick coat does not polymerize evenly, and it leaves a blotchy, sticky finish that flakes later. Wipe the pan until it looks like there is no oil left at all -- there is still plenty in the pores.
Let the pan cool in the oven. One round produces a usable surface, and three or four rounds build a genuinely slick one. After that, cooking maintains it.
Ventilate the kitchen. Oil at that temperature smokes, and the smoke is unpleasant. If you keep birds, move them to another room with fresh air well before you start, since birds are extremely sensitive to fumes from any overheated cooking surface.
Habits that keep a pan in good shape
Store it dry, uncovered or with a paper towel between stacked pans so moisture cannot get trapped. A lid left on traps humidity and starts rust in the middle of the cooking surface.
Cook fatty foods in it regularly. Bacon, eggs in butter, and shallow frying all deposit more polymerized oil and reinforce the seasoning.
Save the long tomato braise for stainless or enameled cookware. Acidic simmering for an hour will dull even a strong seasoning.
Never put cast iron in the dishwasher. That is the same rule that protects copper cookware care and nonstick cookware rules, for different reasons in each case. And do not leave a wet skillet sitting in your stainless steel sink, where it will also cause rust marks on the sink itself.
FAQ
Can I use soap on cast iron?
Yes, in small amounts and briefly. Modern dish soap is a mild surfactant and will not dissolve polymerized seasoning in the time it takes to wash a pan. The old warning came from lye-based soaps that no longer sit under kitchen sinks. What genuinely harms cast iron is soaking, not soap.
How long can cast iron sit in water?
As little time as possible. Rust can begin forming within a few hours on bare or thinly seasoned iron, and overnight in the sink will almost always leave orange spots. If a pan does get soaked, scrub, dry it on a burner, and oil it right away.
What oil is best for seasoning cast iron?
Any neutral oil with a reasonably high smoke point works: grapeseed, canola, sunflower, or vegetable shortening. Flaxseed oil creates a very hard layer but tends to flake. The oil matters far less than applying it in an extremely thin coat and heating it past its smoke point.
Why is my cast iron sticky?
Too much oil, or oil that was not heated hot enough to polymerize. Scrub the tacky film off with coarse salt or a chain mail scrubber, wash, dry on a burner, then re-oil using a much thinner coat and a full seasoning bake.
Can I cook tomatoes in cast iron?
A quick saute is fine on a well-seasoned pan. A long acidic simmer is what causes trouble, gradually stripping seasoning and sometimes giving the food a metallic taste. Use stainless or enameled cookware for sauces that cook for an hour or more.
Is enameled cast iron cleaned the same way?
No. Enameled cast iron has a glass coating, so it can be washed with soap and water, soaked briefly, and does not need seasoning or oiling. Avoid metal scrubbers and sudden temperature changes, and do not use it over high dry heat, which can craze the enamel.
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