A phosphoric acid poison that I like to use for removing rust
chemically is naval jelly. It is a pink gel that you apply with an old
paintbrush. Don't take a whiff or you'll nose will not feel good for a
while. Paint the stuff on the rust and then wipe it off with an old
rag. I use it on machine ways and milling machine vises, etc where I
want to remove light rust but mechanical grinding or wire brushing
would destroy the ground and hardened ways. The scratches left by
abrasive removal also have a tendency to trap more dirt and rust in the
future.