I own a restoration shop... I have used the very guns you have (along with Binks, Graco, and others). They are obsolete. Lots of overspray, higher volume of air needed to spray, and lots of wasted paint in the bottom of the cup that can never be reached.
I have used the HF guns to spray POR-15, High-Build Primers, Epoxy, Enamel, Urethane BC/CC, etc. . They work just fine. Some paints I don't even bother cleaning them-
Especially for POR-15, you will pay more for the gun cleaning solvents than the gun costs, and spend 1/2 hour of time you could be doing something productive.
You will need to PRACTICE... with whatever paint you intend to use. Vertical panels (runs).. so you can learn how not to put too much on, and get an even wet coat.
Drove this 30 miles today on it's first "shakedown cruise".