Pretty much the same amount of work no matter what the cause by what I'm seeing.

It'll have to go to bare clean metal and address what is or isn't there.
On the original seam it was leaded over an open spot welded lap joint. In harsh exposure over time moisture finds it's way through the seam under the lead and creates rust.
Best way to fix it(IMHO), is to remove the factory lead, clean up any rust, weld the seam solid, then re-lead it.

In this case you may even find hole(s) in the underlying metal that will require a metalpatch.

Rick