Rust this will sound stupid but a picked up a civil war colt army 1860 at a local yard sale about 3 years ago
thing was so rusted the cylinder was frozen
A guy said Soak it in Molasses , hot water - in a sealed Tupperwear container it will eat the rust off
I gave it a try half bottle of molasses ( tupperwear shoe box size ) flled with hot water
sealed the top
2days later opened it There was a perfect outline of the rusty section on the piece in the top foam
Frozen pieces freed
seems like the molasses as it furments seeks bases of oxygen since the container is sealed it eats the oxygen from the rust