In the first paragraph of my last post I have a typing error...in the last sentence it should read...width wise, not with wise. Sorry about that.
Also, if you have extensive areas with filler, it's a good idea to "walk the panel", by that I mean don't concentrate on one small area, sand the panel by moving your board across the panel, then change angles on the way back. That way you are taking similar amounts of filler off the panel and have a much better chance of getting the panel straight. Remember that there is nothing wrong with filler, if it's mixed properly, applied properly and the depth of the filler is kept to a minimum. When you mix filler with hardner, fold the filler into itself, don't stir the hardner into the filler. If you stir hardner and filler to mix it you will end up with air pockets and when it hardens they turn into pin holes.