Reasons for various airpressure settings???

At lower pressures there's more control and less overspray but you may need to thin your paints more so the brush can still atomize the paint. As a result you may need to do more coats. Higher pressures atomize better and with thicker paints. I tend to use between 8-15 psi and I thin my paints a lot so I can get the best control/little overspray as I do mostly 1/72.

Chris,

I agree with plum1030's remarks but I would like to add a bit more if I could.

QUOTE: For the most part I cant see a difference when using the VL at various settings while using properly thinned MM enamels and Testors or PollyScale Acrylics.

You won't notice much difference in spray pattern with various pressures.
The main difference is as plum1030 said regarding controlling overspray.

QUOTE: Now when using the Omni since it is a one part(needle/tip)setup, I find that using a lower pressure setting allows me a very fine line, but thats about it.

What do you mean by, "but that's about it?"
Are you saying the Omni will only spray a fine line and not a wider spray pattern, or are you saying it sprays a fine line with little trouble?

QUOTE: So when do you find you need to play with pressure settings, what pressure do you normally spray at? For me its really been between 18-21 psi....

As was mentioned, adjusting the pressure is dependent on the thickness of the paint, but I would say that for MM paints mixed 2:1 through my Omni 4000 and my Vega 1000, I spray between 15-20 psi normally. I may drop it down to 10 psi if I need real control in small areas such as 1/72 scale aircraft as plum1030 mentioned.

Now when I spray my Createx acrylics for T-shirt airbrushing I spray at between 60-80 psi to get good atomization and penetrate the material's fibers better.
That is a bit too high for models though.

Mike