I know many will disagree, but I see no issue using fiberglass and filler for sheet-metal repairs, like behind the battery. Even if you could weld, you might not have access to the backside, which would then later start rusting (weld-thru primer helps). You could also catch the car on fire. Cut back to sound metal, prime well, then fiberglass (woven matt works best). I like Rust Destroyer (converts rust, etches bare metal). Many promote POR-15 ($$$). If totally shiny metal, a self-etching primer might be best. Car shows put Bondo or fiberglass direct on bare metal, but I think primer first is better.