The words of someone I respect:

As someone who has been using body filler most of my life...
1. You can knockoff the majority of the excess filler as soon as at tacks up enough for a surform file to cut without clogging or skipping.
2. Let the remaining putty sit AT LEAST 3 days before final sanding. Scuff the top off with 60 grit twice a day. You want the shrinkage and outgassing done before you final sand and start the primer/glaze coat process.
3. Make sure you coat the putty with an etching primer for maximum adhesion.
4. Rustoleum auto primer over the etching prime. Fills the micro pits better than lacquer based primers. Is lacquer safe, gives a smooth base for the topcoat to adhere to.
5. Wet sand the primer to 600 grit before you add color coat. 2 dust coats of color. Wet sand with 400 or 600, add a "wet coat" of color. Make sure you use a clear over the color.

Have fun. Take your time. When you finish the fender top, but preferably, before you finish the fender top, buy a can of under coating or bed liner spray (Wal-mart stuff is fine). Will protect the topcoats from junk hitting the underside of the fender. Lessens the impact and lowers the chance of the force cracking the paint.
-WD