As with most things, the quality and longevity of great results can be effected quite a bit just by how some of the simpler and overlooked things are done. One of these that can end up being very important is how primer is sprayed.
First, the only primer I use is polyester. With the high caliber of work, and the quality results that are demanded, polyester is the only primer that is stable enough and can be built up safely enough to last a long time, and be a great foundation for a quality paint job.
Polyester is not made for direct to metal applications. It's possible for it to adhere decently sometimes, but it's too much of a gamble. If it is a so called direct to metal polyester, it more than likely compromises something in order to be more general purpose. Nothing is better than an epoxy primer, and then a polyester primer. Epoxy only needs to be sprayed on bare metal areas. **Only use a light coat, just enough to gain coverage of the metal. The epoxy *needs* to be dry for at least 24 hours before polyester goes on top of it. It also needs to be lightly sanded before polyester. This is very very very important.
Spraying polyester. Let's assume all the panels are still on the vehicle. Cleaning is first. The surface needs to be blown off very good, to ensure there is absolutely no dust in any pin hole, no matter how small it is. Then use a tack cloth to wipe the surface off. Wax and grease remover isn't the best idea, just incase the filler holds some of the solvent and it could get trapped under the polyester.
Spray the vehicle as if it is one big panel. This means not spraying each panel individually. Walk the length of the vehicle while spraying. If each panel is sprayed, you end up with over lapped areas, which means there will be 8 coats of primer is some spots when there's only 4 everywhere else. This results in a lot of unnecessary block sanding to correct it.
The first coat should be sprayed pretty light. It needs to stick very good to the filler, and let the second coat stick to it really well. It can't be too dry, where it become like little pebbles either. The remaining coats would be medium to medium/wet coats. Do not spray too heavy. Just because polyester can be built a lot, doesn't mean it's a good idea. Do Not abuse polyester primer!! It can and will crack.
Flash times. Polyester is a little tricky to tell when it's ready for the next coat. I usually spray the next coat when you touch it and it feels pretty tacky yet. It should not be wet, it is too soon for the next coat then.
Do not force dry polyester!! It dries fast enough and hot enough on its own. If you throw heat at it, the chances of cracks or other failures go up quickly. Let it air dry. After the polyester is dry, then it's ok to bake it as much as you'd like. As a matter of fact, I like to bake it a lot, and after each grit is sanded. The more heat cycles anything goes through, the better the end result will be.