Body filler is tricky.

Put it on wider than you think it needs. Don't try to get it"flat" at first. Stop with the 80 grit when it looks close. Shoot it with some rattle can primer then switch to finer grade paper. The grey primer will help you see where it's low. Don't be lazy.. Put down the D.A. and grab the soft block.

The hardest part of filler isn't putting it on... It's NOT taking too much off.