You'll want to get it so that you won't have any spots of filler thicker than 1/8". 1/4" thick is too much, and will most likely crack. I do beleive your father in laws freind is on the right track, that's how I've seen it done on some of the assorted shows on TV. I would suggest popping for the pro-grade fillers, like Dyna-Tron or Evercoat, rather than the regular Bondo. You'll get better results and they seem to be easier to work with.
