Once the panel is sanded down and you start hitting bare metal, this is the time to stop sanding that area and start thinking about either very lightly tapping these bare spots down slightly, or if you may need another blast of body filler. In our case the bare spots that showed up after sanding only needed a little bit more persuasion to be straight. After some tap-tap here and there, we applied one more very thin skim coat of filler. If you are still having trouble getting the panel straight after doing all of this, try spraying a guide coat directly onto the panel. Then lightly sand the area and watch the bad spots come to life. Keep repeating these steps until the area is ready for primer.