I've used BMW's kit for fixing scratches to good effect, even on some rusty spots, albeit less than 1/4" in size. Anything as big as you say requires a can and sanding, as well as rust treatment, which I have also done. Some I must redo properly. Except perhaps for the long scratch I should say, from your description. In that case, the brush would be handy indeed, because the one in the BMW kit is not the best.