I recall that the topic of windshield rust was covered extensively in
the T1N section of Sprinter Forum. You might look for pointers there.
We have a constant battle with roof lap welds on our 2006. The Sprinter body flexes just enough for paint seals to get broken. The secret is to not allow it to become too advanced.
We also have a few perforated areas on the side body (e.g., fresh water connection) where Airstream did insufficient sealing during installation of the appurtenances. Eventually we will have those areas cut out, patched, and refinished. No way around it.