Here's a link to the article on feeling panels for imperfections. The basic tenet is that you can't rely on methodical blocking and guide coat alone to achieve a flat panel - it has to feel correct to be correct.
http://autobodystore.com/forum/showt...ywork&=feeling
After feeling the panel, I'll mark where I think the irregularity is with a pencil then go back and sand with a block using 80 grit to contour (I always use a fresh piece of sandpaper when I get to this step- dull paper can introduce waviness and actually make it worse ) I keep repeating until the defect levels out and it usually takes a few iterations of chasing the imperfection with the block before you can't detect it anymore with your hand. I follow this with a dusting of guide coat and lightly block the entire panel to blend everything together. Then another dusting with guide coat and block with a light touch with 180 until the scratches are gone and you're ready for primer or Featherfill.