I have removed old paint in areas the will need some body fill
(BONDO). I am using krylon rust proof primer and going
back over that with duplicolor primer-fuller. How will this hold
up? Am I wasting time or is this a viable way to prep. for
In my opinion your paint is only as good as the what's underneath it. You wouldn't start with a marginal foundation when you build a house so you probably shouldn't skimp on the materials used for the foundation of your paint job either. I mean why would you dump tons of money in a paint job only to have something go wrong later on because of poor quality primers and surfacers? Lifting and cracking paint in many cases is caused by cheap materials used in the prep process or improper prep methods. This is why many paint manufacturers will only warranty the materials if you use there paint system. This means the metal prep, primers, sealers, top coats etc.
I always say do it once and do it right. Of course this is just my opinion. After all the work in preparing a car for paint you don't want to worry a year or so from now that something might be compromised because of spray can primers...::