read all of martinsrs' article it's good stuff. i've been welding proffessionally for twenty yrs. & agree with 'bout everything he says.
sounds to me like you're not cutting out enough rust. sheetmetal can seem pretty solid & still have alot of rusty pits in it, giving you lots of burn through & pinholes. clean off ALL the rust, old paint, grease, tar, undercoating etc. then your "old" metal will be just like the "new" metal.