To remove rust from the cast iron, you need to soak the rusty iron in vinegar and water solution. After that, use a bristle brush, and check if the rust comes off. If it does not, then let it soak for some more time. But, it would help if you remembered that soaking it for more time does not always mean it is good. Once the vinegar and water act on rust, it will act on the iron itself if you do not take it out of the solution quickly. Once you scrub it thoroughly, rinse the iron with clean water. Before you decide that it needs another round of vinegar water, let it dry completely. Then, you can repeat the steps until you feel the iron is sufficiently clean.