When I picked up a cheap 870 Express for 3gun and discovered that it developed surface rust the just on the drive home from VA humidity, I ended up giving it a couple of light coats of Krylon. It's been 12 years and that coat is still holding up, and personally I think the adhesion and protection seems to be better than similar stuff I did with Dupli-color rattle cans, although the DC stuff seems to he thicker. In short, if all you care about is some degree of looks, perhaps reducing surface glare, and primarily rust protection, I'd Krylon it some flat color and call it a day.
I completely disassembled the gun, hosed off the the outside with brake cleaner, then put paper towels into the cavities, tube ends, and barrel ends. When I needed a fine line some masking tape. I used a cardboard box as a paint booth, and let it dry an hour between coats. Finish ended up super-thin, and super durable once cured.