0000 steel wool and a good solvent is the best way to remove rust. Whether it damages the finish or not is more determined by how long the rust was left untreated, and if any salts were present, and what type of finish the gun has. An 870 "Express" does not have a parkerized finish. It is just a bead-blasted blue and not very resistant to rust. A real parkerized finish is much more resistant to corrosion and wear.
