Well, I gotta jump in on this one. Although I do not have a lot of experience spraying automotive type paints, I have been spraying cellulose and catalytic lacquers for well over 30 years, so I think I just may qualify to add my 2 cents.
Firstly - always buy the very best compressor you can afford. After I opened my cabinet shop back in the 80's, I could not afford the compressor I wanted (still can't), so I ended up purchasing a Pony. This is a nice little compressor that will pump out enought air to do limited functions sufficiently, but it is a struggle. At 40 psi, the compressor will do a comfortable job even spraying, but it will run continuously which is hard as can be on these little compressors. Personally, I would never use it again to spray squat - the finish is passable but not top grade.
The main problem here is cycling - there is a surge of air which translates into bubbles and excess material being forced through the gun (negligable, but noticeable), and at the end of the cycle there is a substantial drop in pressure before the next cycle begins. This causes a loss of air, hence pressure at the nozzle, hence less material being ejected from the nozzle causing misting on the finish.
Soooo, the solution was for me to buy a DeVilbliss 5hp compressor with a 100 gallon laydown tank. This solved all my problems, but is still not the compressor I always wanted. It is missing the dryers and a few other goodies that I consider advantageous, but not mandatory. This compressor can run ALL of my air tools and my spray guns at the same time without a noticeable drop in pressure, not that I have that many hands, but when I had employees, it was a real boone.
Oh, a word here on buying compressors. You can go to TSC or HF or almost any store and spend little or a lot and end up with a low quality or middle of the road compressor. To prevent this, let your fingers do the walking. Investigate your yellow pages. Look under Compressors or Air Tools. Find a business that sells comercial compresssors and also services them. Then go shopping. They usually will have well used but reconditioned commercial compressors for the same or usually less than a Campbell Hausfield or a Coleman or any of those other so called Hobbyist compressors. Then you will have similar to what I have. I bought mine used and it has served me faithfully for over 20 years!
So with the air pressure problems solved we can go on to guns.
The first gun I bought was a DeVilbliss 312 - over $650.00 Canadian. Top flight gun no doubt, but beyond the reach of the hobby painter and is overkill even for pros.
A couple of years later, I needed a cheap gun to do some catalytic lacquers with embedded pigments and did not want to ruin my good guns. So I tried out the Chicago brand of guns. Big surprise
and I do mean a big surprise. This gun was as good if not better in some respects to my 312 which blew me away. Since then I have never spent more than $40 - $50 on a new gun. Some of the cheap guns that are available at Princess Auto and I imagine Harbour Frieght although marketed under different names are produced by the same manufacturer as the Chicago brand. I assume they are Chicago's cause all the parts are a perfect fit and can be interchanged without worry. I know this, cause I have about 8 or 9 guns and I never worry about which parts fit which guns.
There are a few factors that determine how a finish job turns out.
One is finish material quality. Buy the best you can reasonably afford and stick with that manufacturer/brand. This includes Paint/Lacquer/Clear Coat, Reducer and Hardener.
Two is air - buy the best compressor you can find. Oh, and I am not a fan of HLVP -- no where near the quality of a traditional compressor, but then I am kind of stuck in my ways.
Three is median humidity levels. I always try to spray in a 45 - 65 % humidity environment. Not much more as I find the tendency for fish eyes and orange peel is increased. Too much less and the finish will become misty due to the lack of moisture.
Four is dust - keep it down - wet your floor lightly if possible.
Five is operator experience. This is a fluid factor - the more experience the better you will get. Take your time, make sure that you have set up your spray pattern properly, the volume of air to material is correct, your overlap is sufficient yet not excessive and pay attention to your stroke. Stroke is very, very important. Master this and you have 90 percent of spraying all locked up.
Remember to watch out for sags!
I imagine there is a few things I have missed, but I will leave that for another time when I can think of it or to others with more experience than I. Hope this has helped a little and not muddied up the waters too much