We've only had our car for about 4 months and it was new when we bought it so it hasn't started rusting yet. However, it was drenched under the seal and I can see how this is a major area of concern. In order to prevent any future rust I am going to clean up the area very well, tape it off, and then undercoat the seam. That should sufficiently seal the seam so that the rust never starts. An extremely useful thread Andrew, thanks.