In an ideal world, you want three spray guns, one for primer, one for color and one for clear. Superior cleaning of the spray gun is essential to get a non contaminated mix.
Acrylic Lacquer is very easy to use and dries well. Best temp to spray should be above 55-60F if you are spraying outside without a booth or heat lamps.
PPG has a nice system, but there are many others out there.
When spraying metal, you'll want a metal etching rust proof coat, followed by a primer. Then it's a matter of "x" number of color coats followed by "X" number of clear coats.
Fine sanding between coats followed by cleaning the dust (wet sanding) ensures each coat layers properly.
Also the psi you spray with primer is different than color as well as the way the paints are mixed for good flow.
Waiting anywhere from a week to a month before clearcoating as well as optimal times between coats is important as well.
That's a basic overview of what I remember from when I spray auto paint a decade ago.
I'm currently restoring and rebuilding a private motorcycle collection and next year I'll be back to spraying.