Put it in an electric oven and run the self cleaning cycle, clean the rust off with water and scotch pad, reseason.
This ^^^^^
I used the self-cleaning cycle method to remove built-up old seasoning and baked-on hard grease. The high temperatures cause everything to flake off except the rust.
Let the pan cool down in the oven before removing it, then try scubbing the rust and flaking off. If that is enough, then great. If not, then you might have to use a softer wire wheel a bit.
Then scrub it and then re-season.
