Can you season over some thin flash rust and get a usable pan as a result? What are the downsides of doing so?
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 &quot;naked&quot; again.
Additional to that, in any kind of applying coatings (in the kitchen and outside), &quot;hiding&quot; pieces of other stuff under a coating (even if it is &quot;just&quot; 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.