Paul, it looks like you've gotten a good start on it. Nice looking interior on that.
Here's my weathering toot on my Hetzer that has made the rounds -- Hetz Weathering
It shows one good way to do chipping with a sponge. Other than that just use a small pointed brush and go sparingly. Use a mix of black, steel, and raw umber oil, or Testor's "Rubber" enamel, to get a metallic brown color. You can also use a flat brush and "bounce" the flat edges against the side of the model to transfer the paint.