Right on the money Ray, good point you bring up.
Yep, coarser paper like 120 or even courser with the filler of course, using NEW, SHARP, QUALITY paper and changing it OFTEN is the key to what Ray is talking about.
You need to CUT the tops of the high spots off with NEW, SHARP, QUALITY paper. Not using NEW, SHARP, QUALITY paper a lot of times you "polish" the surface instead of CUTTING it flat. The sharper the paper the less pressure you have to apply. This eliminates pushing the panel down "flattening" it out as you sand. I have blocked stuff like a flexing hood where I literally hold the board off the surface so as to not apply any pressure to flex the panel.