Rich,

I had seam rust at the same location.  I sanded down to bare metal along the entire seam, picked the seam clean and filed to bare metal with a small file, applied the black rust inhibitor, two coats of primer, three coats of finish color and a clear coat.  It lasted about two years - the source of the moisture was from the inside.  I now have a mess as the seam is starting to cme apart.   I am also interested in how to get after this and would be interested in the responses to your request.

Good luck.

Ed
84 Westy