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How to paint cast iron 
Remove any rust on the cast iron. 
You may be able to use a wire brush to scrub off the rust. A sandblaster or rust-removing chemical products can also be used if you need to take off a lot of rust and are not worried about possible damage to the cast iron.
Wear proper safety equipment if working with a power tool or chemical to remove the rust. This may include gloves, goggles and a respirator.