DO NOT SPRAY PRIMER OUT OF THE SAME GUN AS YOUR FINISH PAINT!!!
Primer takes a bigger size outlet then your finish paint.
Primer will ruin your good gun especially if you are spray high build primer. I started painting the old way which worked great for years until I got pickier.
I would use IH primer and IH 2150 as per directions with a good gun ($50-$75).
HVLP guns are not really made to spray enamels, they are better at base coat/clear coat.
My HVLP gun has about 50 lbs of pressure to the gun and has about 15 to 20 lbs at the tip. I did buy a good midrange gun ($125)
I am not a great painter but the Scout I painted 2 weeks ago won First place for Scout 80s at the Midwest Show at Red Power.
When I want a Cub to look original I use IH Paint because I can get an eggshell finish like they had originaly.
Painting like anything else takes practice. With a little practice you can do a great job. With years of practice (every day) you can do show cars.