The 870 Express finish will most certainly rust. One weekend in early March my cuz and I went down and tried to do some hog hunting. I was carrying a Marlin and he had my 870. After a long day of trudging through the rain, we decided to pack it in. When we got back to the truck in the pouring rain, I wiped my Marlin down as best I could with a dry towel and did the same to the 870, then put them in their cases and went home.
The guns didn't get wiped down until the next afternoon. When I took the Marlin out of it's case, there were some very light specks of surface rust that disappeared quickly with a touch of break free and steel wool. When I took the 870 out, I was shocked at how bad the rust looked. At the time I thought that their finish was rust-proof, but now I know better. Steel wool and break free took care of it, but it looked pretty bad when I first saw it.