Cast iron utensils will last almost forever if cared for properly. If exposed to standing water or left moist, they may rust. Cast iron kettles and other container vessels are especially prone to rusting if any water or other moisture is left to stand in their bottom when the kettle is not in use. Keep rust from spreading by removing it immediately and then, next time you use your kettle, wipe it dry before storing it. This will help keep the rust from coming back.
Scrub rust spots away by hand with a wire brush. You can also use a rotary tool or power drill with a wire wheel, you can also use them; just make sure to apply the rotating wire wheel very delicately to the cast iron surface so that you grind only deep enough to remove the rust.
Soak any remaining rust spots in Coca-Cola until they're gone. This may take as little as 5 minutes or you may need to leave the cast iron kettle to soak overnight.
Rinse your cast iron kettle clean with warm water. If there are any stubborn rust stains remaining, you can spray them with oven cleaner, wrap the entire kettle in a heavy-duty plastic garbage bag, seal the end off with tape and let sit overnight.
Wash the kettle clean with warm water.
Wipe the kettle dry with a clean, absorbent cloth.
Apply a thin coating of vegetable oil to the entire surface of your cast iron kettle. Wipe off any excess. This will help protect it from rusting again.