Fuck what they say about protecting against moisture. Water didnt even TOUCH my 870 and it RUSTED TO SHIT. I have never fired it and have owned it for about a month. It was living in my trunk in a case and it had rained a few times here unexpectedly while I was at work. No water ever touched it and now it is fucked all to shit. time ti have some fun with a wire sponge and Hoppes No.9. I hope to god I dont have to get this thing re-finished. That would make me even more angry. Pics possibly to come.

FTR, its an 870 Express Magnum 18"bbl 8 shot mag. Rust is found on left side of receiver and on part of the mag tube.

The case you kept it in is your problem. Moisture got in(even micro ammounts cause rust) and held it against your gun. Never, ever keep a gun in a case! My dad made this mistake with a $5,000 1964 Browning Superposed Lightning. Opened the case and it was rusted to hell. Made me through up!

Just get it refinished or better yet guncoat it yourself.

Max

First off, unless you had the gun refinished, it WASN'T parkerized.
The standard factory Express finish is a bead blasted blue.

In order to protect an rough finish like parkerizing or the bead blasted blue, you need to either "soak" the finish with a good rust preventing lube, or seal it with a coat of wax.

To soak the finish, spray the gun dripping wet, inside and out with CLP Breakfree, which is one of the best rust preventing lubes made.

Allow to soak for 20 minutes to dissolve the old factory preservative and to impregnate the finish with Teflon.
Wipe off the excess and you're good to go.  Whenever you handle the gun, or before putting it away, apply a thin coat of CLP to neutralize finger prints and restore the Teflon coating.

Many saltwater duck hunters apply a heavy coat of wax to the gun, warm it up with a hairdryer, allow to cool, then wipe off the excess, leaving a sealer coat of wax.
Some buy a hard carnuba "museum" quality wax, or use ordinary Johnson's Paste Floor Wax.

In either case, NEVER store a gun in any type of fitted case, whether it's lined with a "rust preventing" material or not.
All gun cases are notorious for absorbing moisture from the air and trapping it with the gun.
The problem is made MUCH worse by storing it in a car trunk.  
Car trunks are one of the absolutely WORSE places to store a gun.

If you have to store in the trunk, buy a hard case that's filled with CLOSED CELL foam, which doesn't absorb moisture from the air, and either wrap the gun in some Vapor-Paper, or put some vapor barrier chips in the case.

www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=1197&title=GUNWRAP?%20PAPER

www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=1207&title=RUST-BLOX+VAPOR+TABS

www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=1154&title=TRIPLE+TOUGH?%20PREMIUM%20STORAGE%20BAGS

I went on a 2 day backpacking trip in a national forest in CA with a 870 Express HD.  The first day was it lightly rained all day.  I awoke to find a fine coat of rust on practically the whole gun.  Luckily, I had some vasoline lip therapy that I used to prevent rust on the moving parts.  I ended up selling that and buying a 590a1 parked.  Hopefully that will help.