Amelia,
Having started airbrushing over 25 years ago on t-shirts then to hard surfaces (still doing both) I can related to your frustration.
I still regularly use my first airbrush, a Paasche Vl. It is now highly customized to suit me. I do have other brushes I use also, many among my 30 or so airbrushes are set-up for "specific" applications.
The first thing you may want to try on your ol' Paasche, especially if you switch to a paint more suitable for automotive work, is to put a #1 tip/head/needle assembly into it. With a little practice you will be surprized at the detail you can accomplish. You can try the same with the Vega also.
My recommendation is to start with air pressure and paint selection. Shirts asorb a spraying pressure of 35-60 psi whereas a hard surface has no asorbtion, hence many a "splatter" effect. The excess air has to go somewhere! Set yourself up a practice panel such as a blank license plate. Start off with and air pressure around 28-30 and make sure paint is thinned or mixed per directions. Practice your basic strokes. If you are still having a problem drop your air pressure a couple of pounds. Keep doing this until you find a pressure you are comfortable with without it being so low you are getting a grainy line. I must add that for me I found that I needed to "speed up" some on hard surfaces as compared to shirts. Another thing that I found that helps me (I do this when lettering shirts) is to paint the line or stroke without paint, usually a few times. Simply go through the motion using air but no paint, This helps you to get "comfortable" with what you are about to do. And when you feel ready you simply flow through the motion since you have already practiced it and don't need to think about doing it. (I hope this made sense! LOL!)
Use a paint formulated for exterior hard surface applications such as Auto Air for learning purposes then move on up to automotive base coats, toners, and clears. Even though your water based shirt paints can be used with very good results and success, their durability is far less than desired, even when clear coated. The clear coat offers some protecttion but it still is setting on a soft base and eventually it's protection of the paint will break down due to an unsolid base. Kinda like pouring a concrete slab on mud.
Sorry for getting long winded! /ubbthreads/images/graemlins/bounce.gif
Have fun! /ubbthreads/images/graemlins/yay.gif