I am a little lost, I have not read this thread but I do commercial painting - behr marquee is an interior latex, flotral is for smoothing out brush marks in the paint. I have never put interior latex on the exterior of a vehicle.
If you are spraying you should not need flotral at all. It actually slows the paint drying and allows it to level better and thus the brush marks disappear. water to thin it properly if it is water based and you can use flotral but it will actually cause more runs when sprayed.
penetrol is for oil based but does the same thing, it is meant to level brush marks and is not really needed when spaying. You need an oil base thinner appropriate to the paint you are using in addition to the penetrol, normally it will not thin it enough, it mainly slows the dry time.
Slowing the dry time hurts you on a vehicle especially if you do not have a paint booth. The longer it is wet the more crap to stick to it.
If people are using latex paint with success on their vehicle it is new to me, I just wanted to pass on the info about flotral and penetrol.
I don't know anything about the HF guns
good luck
rich