Boy am I glad nobody told my Mosin M44 that guns can't stay in the trunk without getting rusty!
Before changing my mind about "trunk guns," I had it in the trunk here in the drastic temperature changes of New England for about 2 years.
I had sprayed it up with Eezox, and in two years, without any maintenance, it showed absolutely no signs of rust at all. And that's just a straight blued gun.
Each time I'd check on it, it still had that sweet, kinda cherry, smell of Eezox, even two years later.
Seriously, Prax, your shotgun is a tool. You obviously weren't overly concerned about it in the first place if you put it in the trunk, so why bother getting rid of it, or getting it refinished now, unless of course, the rust is effecting function?
I'd strip it down, use some penetrating oil and wipe off as much rust as I can with a course towel of some type. Some guys like to use steel wool, but I avoid it, as it introduces carbon steel to the surface of the gun, and leaves the little fibers behind which can then rust and make things worse.
Disassemble, use a really good oil that will stay on there, reassemble, test-fire, clean, and throw it back in the trunk if you want. Who cares if the cosmetics are FUBARed? You didn't buy it as a museum piece. If it works, use it.