My favorite rust remover is cola. Just buy a two liter bottle of generic cola and pour it over the rust and leave it for a few hours. When the time is up, use some steel wool to remove the rust from the surface.
Once all of the rust is removed, wash the pan in hot soapy water. Normally you wouldn't wash cast iron with soap - but during this cleaning process before you clean it will be ok.
If the buildup is really nasty, you can place the pan in your oven and run it through a self cleaning cycle and then let it cool completely before washing. This will burn off any rust and residue.
To season your cast iron, set the oven to 250 degrees and coat the entire pan with some solid shortening.
Bake for fifteen minutes and then remove. Carefully wipe off any excess grease and then pop it back in the oven and bake for another hour or so, cool completely and you're set to go.
A tip - if you put a paper towel or a coffee filter between your pans it will absorb excess moisture and prevent rusting! I'm Mrs. Fixit and it's just that simple!
