Not exactly primer but along the theme of the thread.
The truck is too hammered and rusty to do anything serious with. I did a two day scuff and shoot. I used a 5" ridgid orbital sander and some 180 grit to get the big stuff and some 220 by hand over the whole truck. The primer is summit racing white sealer. The paint is rustolium 50/50 mix of gloss white and flat white. I used a 50/50 mix of acetone and mineral spirits for the same reasons Mark stated above (spectacular write up BTW).
The top is an implement paint that my shop mate had laying around. I wish I had more time to experiment with it. I shot it with mostly mineral sprites and it took forever to dry.
Steve