This doesn't sound like an heirloom. It's a tool unless you intend to sell it. Sand out the rusted areas. Use a little denatured alcohol to get rid of any gun grease/oil, spray can primer paint on it. Follow up with spray can whatever Rust-Oleum color you choose.Will Cerakote fill rust pits? I found that a single shot slug gun I have has gotten seriously rusted in a closet and I have always thought about painting it so this might be the time to do so.