Stainless steel guns can eventually develop rust overtime. Oxygen can have an adverse reaction with oxygen, forming a chemical compound that is harmful to stainless steel; steel combined with iron, water and oxygen will form iron oxide, or the technical term for rust. Most stainless steel guns--if not too damaged by rust--can become salvaged with the right materials and cleaners.
Items you will need
Baking soda
Steel wood pad, toothbrush or soft brillo brush
Steel bore bristle
Gun lubricator
Soft cloths
Paper towels
Pipe cleaner
Anti-rust oil
Use caution when removing rust from a gun. Dismantle any bullets from the gun and lay the gun on a solid surface, with the barrel away from you, while you are cleaning the gun.
Moisten a steel wool pad with water and dip it in baking soda. If you don't have a steel wood pad, use a soft toothbrush or soft brillo brush as substitutes. Only use materials on the stainless steel pistol that are non-abrasive so that the gun's finish won't get damaged.
Wipe the gun down with the baking soda solution, being careful to not use too much pressure on the gun as it can damage its finish. Use a gun lubrication to polish up the most affected rust areas on the gun. Use a steel bore bristle (located at most gun shops), to polish up the stainless steel. Wipe off moisture from the gun with a fry cloth or paper towel; not wiping the gun completely dry will allows for the water to form more rust in the future.
Remove rust inside the gun with a dry pipe cleaner; using pipe cleaners with the bristly-brush nylon interspersed between the cotton can be most effective in removing rust.
Apply an anti-rust oil with a zinc base to the gun after rust has been removed. You can purchase anti-rust oils from gun shops.