You ain't going to like this, but a bead blaster and a hot blue job. If its just surface rust, you might be able to wipe out most of it with a powder solvent and then throw on some oil. I know from hunting in the Chesapeake bay for ducks, I have some surface rust appear after a day of hunting. And when the day is over, Rem oil spray all over, then wiped off seems to take it off. Thats the reason I take my cheap 870 out there.