Dutch-modeler
...I already got a paasche airbrush single action which is great for big projects like 1/48 bombers for example.
But now I really need an airbrush that can do very precise lines for pre shading...
But the problem is that I couldn’t do any fine lines. I tried everything by mixing the paint, adjusting air pressure etc.
but I think my nozzle is just not capable of doing this sort of job...
You need to provide some more info, before we can really give you advice.
More specifically, which model Paasche airbrush do you have, and do you have the smallest size of tip, needle and aircap or head for it? I have the VL, for example, and the smallest size is #1, which is a .55mm tip, with a matching needle and aircap.
A friend of mine in my club suggested removing the aircap to get a finer line, but I haven't tried it yet. I'm not sure if he was kidding or not.
Also, you might want to ask in the Airbrush subforum, too.