I usually take a graduated approach, depending on how bad it looks, how valuable/important the gun is, and do I want to actually refinish it or just make it decently serviceable. If it doesn’t look too extreme, I’ll generally try a coarse, heavy cloth with some Kroil. Kano labs (makers of Kroil) also have a product called”Ex-Rust” that will remove rust. To get more aggressive, I like Brownell’s Oxpho blue applied gently with 0000 steel wool. Often that process can clean up/cover up spots of rust.
