On an 870 ....any old belt sander and a piece of 80 grit sandpaper ought to work just fine ..../and high speed is probably preferred / and then a rattle can of "krylon" ought to fix it ....
Seriously ( don't do that )... I'm just picking on my 870 buddies that seem to like em rusty and nasty and ugly looking ...
For a serious answer - I've used BoShield Rust Free ... on some of my woodworking tools ..when I screwed up and left some wet wood on a table top or something ... Its a quick acting product so I think it has the potential to help you out.
Whatever you do - go slow ... maybe test the product on a portion of the barrel under forearm / or where the barrel goes into the receiver - where its not noticeable first. I don't know if BoShield will attack a blued barrel or not ?? - but its a very good product. Many woodworking stores carry it ( for protecting cast iron table saw / joiner tops ...etc ....
On this one, there is more way to skin this cat. Kroil is good but there are certainly, no shortage of product that folks use on a regular basis. Lately, I have been using StrongArmBrand and you can google it as I have to order it on line. Great advice on going slowly.
Might I ask if this is a newer 870 Express.
Just asking. ....