You see it all the time, "never use steel wool on a pistol", but up until 1937 the Lugers were rust blued. The rust blue process started with coating the pistol parts with a solution containing nitric acid. The pistol covered in the solution was put in a humidity cabinet to speed up the rusting process. The pistol was taken out, washed in water to halt the process, and the rust was carded off with wads of steel wool. It removed the rust but left the blue underneath. Where the rust has become corrosion, it will have removed the blue in those areas.

The pistol shown below exhibits it's original rust blue finish, and started life being scrubbed with steel wool.