What seasoning actually is
Not a coat of grease. When oil is heated past its smoke point it breaks down and the fragments bond to each other and to the iron, forming a hard, slick, plastic-like layer called a polymer. That layer is chemically bonded to the pan. It is not sitting on it.
Which explains the two rules that follow from it: the oil has to get hot enough to polymerize, and there has to be very little of it, because thick oil cannot polymerize all the way through and the un-polymerized part stays sticky.
The mistake everyone makes
Too much oil. People put a tablespoon in, swirl it around, and bake it - and get a tacky, blotchy varnish that flakes off later.
The correct amount looks like nothing. Oil the pan, then wipe it off with a clean cloth until the surface looks and feels dry. It is not dry; a molecular film remains, and that film is all you want. Depth comes from doing it four times, not from doing it thickly once.
Temperature matters more than oil choice
The oven has to exceed the oil’s smoke point or nothing polymerizes. Grapeseed smokes around 420F, canola around 400F, so 450 to 500F covers both with margin.
A 350F oven, which is what a lot of older instructions say, does not get there for most oils - and is the second-most-common reason seasoning fails.
Flaxseed, and why we stopped recommending it
Flaxseed oil polymerizes into the hardest layer of any common oil, which made it popular after a widely-shared blog post a decade ago. It also produces a brittle layer that has a tendency to flake off in sheets once it gets thick - which is much worse than a softer layer that wears gradually.
Grapeseed or plain canola gives a slightly softer layer that behaves better over years. And both are cheaper and also cook.
Maintenance is the real seasoning
After the first few oven rounds, cooking does the work. Fry something in fat, wipe the pan out, dry it on the hob until it stops steaming, then wipe a smear of oil over the surface while it is warm.
That is thirty seconds and it maintains seasoning indefinitely. People who end up stripping and re-seasoning every year are usually the people skipping the drying step and putting a damp pan away.
What does and does not damage seasoning
Does: long acidic cooks, dishwashers, soaking, boiling water for extended periods, metal scrapers used aggressively on a new layer.
Does not: ordinary washing-up liquid, which is not the lye-based soap the old advice was written about; a stiff brush; brief acidic cooking; metal utensils in normal use.
And if you do strip it, nothing is lost. That is the whole point of a surface the owner can remake.
