only work on the filler you will sand it too low. you need to keep it as high as the surrounding metal, so use the metal as sort of a straight edge that you run the block or board off of.
don't worry if you cut through this skim coat here and there. in fact, you will most likely cut through. the point of that "last skim coat" is that after you add it, you don't add any more filler. that "last skim coat" is just that the last filler you add. if you hit a little filler below, or metal, that is normal and fine. the only thing you are 