I have one large cast iron pot with a lid. I noticed this after getting it out of storage (yeah, I haven't used it for several years for some reason). Anyway, I looked up instructions from the Lodge cast iron company.
First, remove any signs of rust with steel wool or a rust remover you can purchase from the Lodge store.
They say to wash good with soap, and not to worry because it's getting re-seasoned next.
To re-season, spray a light coat with cooking spray and bake in the oven at 450°F for one hour (place foil on bottom rack to catch any oil).
They said it could take 2 times of doing this to correct the pot.