When dealing with Aztek airbrushes, it kind of depends on which airbrush you have. So, which Aztek airbrush do you have? Here are some genearlly guidelines when airbrushing fine lines:
Method 1:
Thick paint (viscosity - high)
High pressure (30 psi and up)
This allows you to paint fine lines when the needle is partially retracted into the airbrush.
Method 2:
Thin paint (viscosity - low)
Low pressure (10 - 15 psi)
This combination produces fine lines with the needle fully retracted into the airbrush.
This information comes from Testors website : http://www.testors.com/hobbyist_guides.asp Click on Chapter 6 and scroll down to the heading Perfecting the Balance of Fine Lines.
I use two Aztek airbrushes, the A470 and the A320. Let me know if you have any other problems.
Jesse