I should clarify my issues with the finish...
If you get it wet, it rusts quicker than any gun I have ever seen. The finish seems to soak up water and develop a surface rust in the blink of an eye. OK not quite that fast, but to the point it needs attention ASAP after getting wet, or even exposure to condensation, which I never really worried about before one of these guns.
That said, if you keep the gun soaked in oil you won't have that issue. The finish will hold the oil well, but it needs some TLC to stand up to the elements.
Comparing to my Mossberg, and Maverick it is a pain to worry about. The Maverick is my Deer gun for the controlled hunt here in Southern Ontario, and it goes through a week of rain and dampness every year, and never an issue. The guys with 870 Expresses all have guns with a red tint to them at the end of the day if theres is any rain.