If they still make it there is a product called alumalead that worked for covering up welds. Sands like aluminum though. The fiberglass reinforced filler will work and so will plastic filler and so will Alumalead. It's really personal preference.
I agree that the fiberglass stuff sands crappy but I would much prefer to use something waterproof (aka shortstrand fiberglass filler) over my welds to prevent any moisture becoming in issue if there are any pinholes in the welds. I just use enough to go over my welds with and then use regular filler for any filling/shaping of the panel. Regular plastic filler is just going to soak up that moisture and turn it into a bigger problem. If metal panels are flexing so much that they are popping off shortstrand filler I would say there are bigger issues to worry about.