Which Epoxy Primer?

I've had good luck with the DP40. And yes, you can use filler either under or over it according to the PPG tech sheet.

Can't answer the question of the e-coat, but will be watching the thread to see what others say about how to prep that.

There's lots of good epoxies out there, so having said that, I'd choose not just the primer, but the whole paint system you want right now. That is, choose a brand of paint that you can use from start to finish. I personally don't think it's a good idea to mix primer from Brand "A", base from Brand "B" and clear from Brand "C". Find a local retailer or internet supplier that you like, and either ask for their recommendation or do some research and make a choice based on reputation, ease of application, customer service and price.

I'm with Zookeeper, stick with one line of paints for the whole painting process. I use DuPont paints, but you can't go wrong with PPG or any of the other major manufactures. I think an important part of the selection is your paint supplier/jobber - if they're too busy to answer your questions and aren't friendly go somewhere else. This stuff costs too much to not get quality assistance from a jobber!

You can spray over the E-coat, just need to sand or scuff it first. I know a lot of people that mix their product lines, including me to some degree. The more important part is to keep within the same line for that particular application, don't mix and match the hardeners/activators, reducers for that type of primer or paint.

I know lots of pro painters who mix all kinds of brands. But my question is why? Why in the world would you spend all that money on material, then second-guess the guys who desgined it to work together? Maybe you'll get away with it, but what if you don't? If you mix brands and it goes to hell, don't expect any help from either the manufacturer or your local supplier. At least if you stick with the same brand and it wrinkles, lifts or peels, they can help you out with advice and may even cut you a deal or just donate more product to help out. When my wife's Corvette got painted with S-W, something went haywire and the clear wrinkled as if it were lacquer shot over enamel. The painter explained the problem to the local supplier, who in turn spoke to the local rep for S-W, and they kicked in some free base and clear to finish the job, which went according to plan. Had he used another brand of clear, there would have been finger-pointing on all sides and someone would have been paying for more product. That one time was enough for me to learn from someone elses mistake.

The why is mostly cost or availability in my case, I use mostly Dupont but it's expensive so I use Omni high build primer, Evercoat Slicksand poly primer and if I want to use a tinted primer I use Sikkens. If you have some sort of failure with it odds are it's from something other than compatibility, like contamination, improper cleaning, etc.. You can't go wrong with staying with one brand though so I'm not advising against it.