I live in a tropical island, 85% humidity, minimum, all year long and a salty atmosphere (as in a island you are never far from the sea, obviously).
The underneath of the def was sprayed from the factory with what I suspect to be waxoyl (or similar).
Since 7 year I noticed very few rust spots, but no rust on the chassis or other important parts.
If you def' gets a reasonable attention, it will not rust more than any other car.
Maybe less as it's simple, without many "water traps" in the bobywork and a aluminium body.
But if you park it in a wet barn after a day in the woods full of mud underneath, it will.
