I have a 1967 Mini Cooper. Along the perimeter of the roof is a drip rail. The roof panel is spot welded to the body in this drip rail (pics can be seen here: http://www.drjing.com/RoofSeam.htm). I have to get a layer of putty or something in there to prevent water from getting into the seam. It has to be somewhat flexible, and be impervious to moisture to prevent rust from spreading any further, but still take paint… Something like a rust encapsulator seam sealer would be ideal.