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I've had a carbon steel pan with some thin flash rust spots. I've tried to remove them by scrubbing, but after applying a new layer of seasoning it turned out I hadn't removed all of it.

While I find a lot of posts online about how to remove rust from pans and how to prevent rust in pans, I can't find a clear answer to the question whether a thin layer of rust under layers of seasoning is actually harmful. The way I see it, once there's enough seasoning on top of the rust there won't be any oxygen for the rust to expand and the surface that touches your food is free of rust.

What are the downsides when you have some flash rust in a carbon steel pan and continue seasoning over it until the pan stops giving off rust when rubbed with an oily paper towel?

Some of the problems I can imagine happening (but I'm not sure if they will happen):

  • The seasoning won't hold onto the pan as well as it would without the flash rust.

  • The new layers of seasoning won't 'grab onto' the rusty parts meaning that you will never arrive at a point where the existing rust is trapped inside layers of seasoning. In other words, when rubbing it with an oily paper towel the pan will always continue giving off some rust.

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    Why not just clean it down & start over? You obviously didn't have a sufficiently thick, impermeable seasoning coating on it previously, now would be the time to do it properly. – Tetsujin Apr 22 at 18:50
  • @Tetsujin yea that's what I'm doing. The question is more of a thought experiment. How bad will a little bit (i.e. most of it cleaned off but just a little bit of flash rust left or reappearing in-between the cleaning and reseasoning) really be? – JJJ Apr 22 at 19:08
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An iron pan is one single piece of substance. It just stays the way it was cast.

A patch of rust is a brittle, powdery substance. It will crumble with time, pieces of it falling off, no matter if they have a bit of seasoning on top or not. So, you will end up with these spots being "naked" again.

Additional to that, in any kind of applying coatings (in the kitchen and outside), "hiding" pieces of other stuff under a coating (even if it is "just" an old patch of the same coating you are applying) is a hallmark of shoddy work. The integrity of your new layer is compromised, and you get quality problems over time. Any professional painter spends more time sanding away old paint than applying new one. It is just a physical property of coating layers that they require an even surface with known-good qualities. If you want a good coating on your pan, you will have to do the stripping work.

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