So far what I have been doing(what the local paint guys told me)
I strip to bare metal, do my bodywork(fillers,icing,etc) Then prime with a filler primer(requires hardener)...this primer is also a sealer when mixed differently. Then I will paint.
Once the primer is dried and sanded it is tough as nails. I like the fact that I don't have to buy a sealer to go over top of it with before paint.
BTW it is a U-Pol filler primer...I was using CarSystems but they seemed to have disappeared.