How far to sand? This is always a tricky thing to get just right. You always want to rough the filler in with 40 grit, and you want it thoroughly finished in at least 80 grit before polyester primer. But when do you switch grits, and how do you know when to stop? I always stop with 40 grit before any metal is showing through the filler. It usually won't quite be straight yet, but it will be in the ball park. 80 grit does a great job of fine tuning the filler, and actually getting it straighter than 40 grit would have. You need to allow enough room in the filler to make sure you can sand all of the 40 grit scratches out (without squeezing filler over them to fill them), but not so far you start under cutting the metal. You want plenty of metal showing through, but none of it should be shiny abrupt spots, it should always be metal, and a hazy transition into the filler. "Hazy gray" you can call it. Those are just a few tips to make sure you have plenty of filler sanded off, but not too much either. And it takes plenty of practice too!