I suppose the paint system that you use could make a difference overall and you might want to talk with the guys you buy your supplies from but in this case my buddies said I should have simply used a good self etching primer over the entire fender first, let it cure, scuffed it with a scuff pad, cleaned it and then applied the Bondo. Then you squirt the high build 2k, let that dry out and shoot a guide coat for finding the high and low spots. THEN you knock years off your life by sanding and sanding and sanding.

I'm not a paint expert by any stretch but I've been told over and over by many different people who are in the auto body/paint business... One thing you should always do is use the same paint manufacturer, or "paint system" for your paint supplies. That way you minimize the chances of running into incompatibilities between primers, sealers and paints that can cause paint failures.