Should work great on a metal seam... As long as you use a very thin coat and top it with some more workable filler. Does a heck of a good job sealing out moisture but not recommended as a final filler... Use a little bit, just enough to cover the welds and cleaned seam, then go to a filler with fine strand 'glass, then the lightweight filler....

All fillers have their proper usage, no one is good for everything.....

BTW, the method I suggested came straight from an Evercoat rep a few years back. Never have had a bit of trouble with it when used in this manner..... Also works great as a first coat (again, very thin) when molding in bike frames, etc....