A few years ago we forgot about our cast iron Dutch oven after
a camping trip and when we got back to it had rusted all over
the inside. At someone else's suggestion I filled it up with Coca
Cola. We were told that only Coca Cola will work, but haven't
tried other sodas ourselves. I let it soak for a few hours and the
rust was very nearly gone. Only a very thin layer remained in
some places that was easy to scour off with steel wool. It
definitely worked better than soaking the cast iron in vinegar.

 

Old Fashioned Living