I'm tellin ya soak it in Coca-Cola, cheap fast and edible.

http://www.iliveoutdoors.com/reviews/features/castseasoning.php

Have a Rusty Oven?

Though there are many methods for cleaning rusty ovens, the one that we have found to be the easiest to use is a can of Coca-Cola. If the inside is rusty pour the Coke in and let it do it's job. Depending on how rusty the oven is will depend on how long you will need to leave the Coke on the rusty spot. If you have an issue with the outside of an oven use a sponge to apply the Cola, or place the cast iron in a large bowl or bucket with enough Coke to dissolve the rust (some rotating may be necessary). After rust is removed be sure to wash and reseason you cast iron.