I seem to have continued success with DOM-16, a Canadian competitor to POR-15, available at NAPA for about half the cost of the POR.
I've done quite a few pieces here, and it's worked well. The DOM-16 is available in black, grey, and white. I hope to interchange it with the accessories from POR. I've been using metal prep, and plan to use the POR primer next, for painting the DOM-16
Not useful for UV exposure, the POR and DOM automotive coatings are single part paint, rather than a potentially even harder two-part like Endura or others.
After proper preparation, the DOM can be applied directly to rusted steel, although I always choose to blast, then chemically remove all rust before doing any coating.