One complaint that I’ve heard fairly regularly on the 870 Express is that it’s blued-over-beadblasted finish is particularly prone to rusting, sometimes quite readily.
In Arizona this probably wouldn’t be too much of an issue, but what has been the experience of you Express owners?
No rust.
Have had one 870 Express or another for just under 20 years.
Same here. This year I’ve spent more on the stock, barrel and mag extension than I did on the original rifle, making it 3 Gun tactical worthy. 
Up until a few months ago it was your average 28" Express with 3 round 3" magnum capacity. I removed the “plug” and shot it in my first 3 Gun match for a total of 5 rounds. 
No rust under normal conditions…
It was accidentally dropped in a fast moving river (the boat I was in mysteriously tipped over) and had to stay there for about 2 weeks. When the water receded and I found it there was substantial rust inside and out. After being cleaned and lubed it has now worked flawlessly for about the last 5years…with no rust.
none here ether
My 870E sits in a closet off my humid master bathroom - no rust.
I’ve had no rust on either of mine from sitting indoors…and the only outdoor rust I got was during a waterfowl hunt at the outlet of freshwater stream to saltwater bay. Salt spray from the boat hit the shotgun and I didn’t wipe it down after the hunt. Couple days later I opened the case and there were a few speckles in the shape of water splatters. Wiped off with damp scouring pad, let dry, and ran light coat of CLP over it. Never came back.
I knew one guy that would apply a couple coats of Johnson’s Paste Wax over all the metal surfaces of his Express before he went hunting. He swore by it as far as rain/moisture protection but it doesn’t seem like it would be a decent long-term protection.
I used to run a light coat of Kiwi shoe polish in neutral over my wood stock before going on a hunting trip. Never thought about applying anything to the metal. 
While I’m not an 870 express owner, I thought that my experience might be valid so here goes:
I once had a new Mossy 590 that rusted like crazy…When I had a swamp cooler. All that humidity being blown in just played hell with it.
When I moved to a house with central AC, it wasn’t a problem anymore.
If you have a swamp cooler, I’d strongly suggest investing in a silicon soaked gun sock as well as checking it at least twice a month.
If you have an AC, I’d simply keep it well lubed & check it only once a month.
My $.02 from a fellow Arizonan.
I own a recently mfg 870 Express made around 2006 and have had no rust problems to date.
Other than cleaning and lubrication it receives no special treatment and has had no rust problems for the past two years.
Yes on rust after shooting one rainy afternoon. It cleaned off pretty easily, but I still have an eye out for a marine shotgun at a good price.
Mine rusted easily and urine discolored the finish.
I’ll make sure not to piss on mine.
I’ve had one since they first came out. Once I got a couple of small rust spots on it I had to continually address those areas. One morning, I got a call from a friend who was hosting a parkerizing party that afternoon. Being so invited, I brought along my 870 express and a case of beer. It came out bueatifully. The only thing I have left on it to complete is an XS front night sight and possibly some type of rear sight. FWIW, the bead blasting smoothed up the action quite nicely since the factory grit blasting is quite aggressive.
I’ve had one for about 9 years and I bought a second one about 2 years ago and I haven’t had rust issues with either of them. I live in Texas.
I had no intention of pissing on mine either. But rain ,cold and hypothermia make strange things happen.