like someone said oven cleaner does wonders. my dad just bought a set of skillets and pots at a auction for 20 bucks. it would likely cost 300 new. They where pretty rust he used oven cleaner and the green pads for cleaning dishes, it all went away. Iv seen him do the same to some worst. id get a big fire going burn everything down till its carbon then scrap clean. then use the oven cleaner. a good coat of oil return to the fire and your good to go.
I normally wash mine with dish soap after every use. Most swear not to, but I dont like cooking something in a unwashed skillets after I cook say chicken. A good through rinse and re season.