Rear quarters already on, has anyone ever put POR15 over EDP, or is there better rustproofing for the backside area, once fenders are on?

Gonna be hard to reach down in there....

Joe

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Joe Beyer
1969 Malibu Convertible (kinda)
http://www.chevelles.com/showroom/convtrailer.jpg

No, don't use POR-15 over the primer. It will peel off down the road. It's not meant for applying over any painted surface.
Spray the backsides with an epoxy paint (black). You can REALLY lay that epoxy on nice & thick and it will adhere to the primer very, very well after a scuffing with 320.