(Without going back 20 pages to see if this has been mentioned before). I think using automotive "Seam Sealer" might be all that's needed to prevent the rust from starting.
https://www.google.com/#q=seam+sealer
But since the rust problem has already started on many Crossfires already, it might be a good way to keep it from coming back after repairing the rust damage.
I don't know why Karmann didn't use it on the door in the 1st. place. They had no problem using seam sealer under the hood on the inner fender shock towers.