You probably aren't drying it right.

No, you aren’t drying it immediately enough — water hides in the porous surface and flash-rusts.

Cast iron isn’t smooth like glass. Even after you towel-dry it, microscopic water is sitting in the seasoning’s pores. That water hits oxygen and rust forms in minutes, no matter how fast you were with the towel.

The fix is simple: after you wash and towel-dry, put the pan on a burner over medium heat for a minute or two. You’ll see steam rise, then it’ll stop. Keep heating until it just starts to smoke. That’s how you know every last drop of water is gone. Then wipe in a tiny layer of oil while it’s warm.

Also: if you’re using soap, stop. Unless it’s a rare modern mild soap, most dish soaps strip seasoning and expose bare iron. Hot water and a stiff brush is usually enough.

That extra minute on the stove saves you from scrubbing rust later.