How do you get rid of rust on cast iron pans? These are rusty inside and out, they've been subjected to moist conditions while stored in an outside kitchen.
Rub them with Coca Cola. The phosphoric acid will eat the rust (slowly).
After they are clean, I would wash them, then dry on stove top.
They should then get a good rubbing with lard (or the fat/oil of your choice).
They may need a reseasoning, depending on how bad they got.
Depending on how bad the rust is, steel wool, sandpaper, and on up to sandblasting will work.
That's the beauty of cast iron, short of cracking it, you really can't hurt it.
The best and easiest method I've ever used is to shove the whole pan (after unscrewing the wooden handle if it has one ) into a hot outdoor fire and let it die down around the pan. THe pan needs to stay in long enough to burn off any carbon from the fire.
Pull it out hot, or leave it, give it a good wash with steel wool then season it. https://permies.com/t/154/cooking-food-preservation-food-choices/cast-iron-skillet-ain-so