It's been over ten years when I took auto body at the local college and if my memory serves me correct this was the sequence of sanding. Is this correct:

40 grit- rough the filler
80 Grit- shape filler
prime-.....find high and low spots
220 grit-smooth the repaired section
400 wet sand- prep for painting

Thanks David

In a nutshell...yup.

do not prime over any sandscratches larger than 180 grit. In fact 220-320 would even be better.

You can prime over 80 grit but you leave the primer with such a big "tooth" to fill you run the risk of shrinkage.
Final sanding filler with 180 leaves that primer with NO chance of shrinking back...Eric