On the older 870 Express guns with the rough finish, a can of WD-40 Specialist Long-Term Corrosion Inhibitor will do the trick. I have been impressed with its performance on my cast-iron BBQ smoker. One application stops any rust from starting for a year, other than on the firebox (burns off). And that poor thing sees lots of rain and condensation. It leaves a white waxy residue on the metal that seems to keep oxygen and water from the metal. Not pretty, but it works.