If I am not mistaken, the underlying primer and etcher used for painting is what gets damaged and it seals the surface against corrosion. It has been a few years since I messed with paint but I remember some things, like don't paint in the rain.
The thing is if you sand it, you got to feather it and apply a 'like' sealer or even a chromate primer, the idea is to minimize the area where the repair is being made and to prevent moisture from getting between the paint and the metal. If it does, no matter what you do to seal it, you will have rust forming on those spots.
I no longer trust dealers for paint work (READ especially Dodge), they have a different attitude since a majority of their work is on lease vehicles and most of the time they will never see the vehicle again, so they don't care to do the job right. I would actually look for a shop that does Mercedes cars, sounds stupid but they understand the painting process and I don't think that they would screw up a high end mercedes product like a Sprinter.