How to do fine lines with Paasche double action and Humbrol paint?

You SHoudl be ok on the humbrol mix...tae it to 60% thinner and give a try. Might wanna try like polly scale sol on it, its a little stronger and has an even break down.

i would drop the pressure as low as i could and still get the siphon to work and thin the paint ever further if needed. its a whole paint viscosity to psi to airbrush to user ratio thats gonna determine the outcome of the line. when i'm doing fine line work i'm below 10 psi, i have my paint thinned at somewhere in the nieghborhood of 60%thinner to 40%paint, sometimes i have the aircap off, sometimes i dont, and i'm spraying inside an inch from the surface with short quick strokes the majority of the time. but i have a gravity feed brush and i'm not familiar with the capabilities of the VL. good luck. later.

It is hard to get paint to flow well with the #1 needle as it was designed for finer pigment mediums such as inks, etc.
I am not familiar with Humbrol paints but I would say that with a modeling enamel the #3 needle should be able to spray as fine of a line as the #1.

Mike