When do you set compressor PSI?

I was fiddling with my new Iwata airbrush last night, and I realized that I didn't know how/when to set the PSI output of the compressor. Until now almost all of my airbrushing was Parma Faskolor on Lexan R/C bodies, which calls for 30+ psi. My little Testors compressor only puts out about 25 psi, so I just left the regular wide open for spraying.

Now that I need to reduce the pressure, I'm not exactly positive when to adjust the regulator: Do I do it when the airbrush is spraying or when it is 'closed'? Pressure usually drops like 8-10 psi after I start spraying.

Good compressor shouldn't drop in pressure like that (unless you have no tank)

anyway, adjust it when you are spraying paint on a paper. you will have perfect adjustment.

Steve, like Eric said, you adjust the pressure before you spray the part but after you have the paint in the airbrush.

OK, so adjust the pressure when spraying whatever medium you will be using, got it.

This compressor is a cheap little Testors, the next step up from the Ice Blue one. I do have a garage compressor with a tank that I have been meaning to set up for some time, just haven't gotten around with it.

BTW- Did you guys know the Medea paint included with some of the Iwata airbrushes sucks when you spray it on plastic?