You’ll want to remove the rust… a paste of warm water and Barkeeper’s Friend, and scrubbing with a stainless steel scrubber will work great on both the rust and the old seasoning. I have also used vinegar (just fill pan and let sit at room temp for an hour or so) as a second step for the inside of the pan.
Just remember to season the whole pan inside and out going forward.
There are of course other methods out there, but these are the only ones I have used and they work just fine.
