Chris and Jim are giving sound advice, as usual. There are many variations on the theme. Here's one timeline that included poly primer, in this case it was a fairly badly damaged GTO fender:
Strip/blast, epoxy, metalwork, sanding (red scuff/220), epoxy, filler, blocking (40/80/150/220/red pad), epoxy, polyester glaze/putty, blocking (100/150/220/red pad), epoxy, poly primer, blocking(100/150/220/320/red pad), epoxy, urethane = ready for final blocking.
These sessions stretch out over weeks for me, since I only get 1-3 hours to work on project cars every several days. Beyond their essential purpose of protecting bare metal and porous filler, the semi-gloss layers of epoxy help show by reflection some subtle things that might be missed with the hand, they also obviously add mils for blocking.
In the method I use for restoration body work, epoxy and polyester filler/glaze/putty play by far the largest part in the straightening process, and the polyester primer step is only needed for seriously damaged panels. In most cases we would skip that step, or replace it with either an additional epoxy or urethane step.
The drawback to poly putty PRIMER in my mind is it can produce some serious build in areas where it's not wanted, like door handle depressions, side marker holes, edges, etc. Care must be taken to remove the film build from these areas, which has to be done with every other kind of primer, too, but with poly it just seems a lot harder to get it back down to where things will fit right and edges won't chip off.