I've been trying to do ths but can't get the thin lines I'd like to achieve. I'm using a 2mm nozzle, low pressure and thinned paint. I can get nice thin lines ona porous surface like a paper towel but on t he hard surface of the model when I get close the force of air pushes the paint away.

Can anyone suggest how to do this to get thin lines?

Thanks.

My suggestion would be to lower your pressure more to under 10 psi or if you can move your airbrush back on the surface just a bit more. Good luck.

Painting lines on paper is much easier as you found out then on plastic, because the paper will absorb the paint, while the plastic doesn't.
For starters, I would prime the model, this will help with the paint staying where you apply it, as the primer will to some extent absorb the paint, and the valleys will hold paint as well.
I wouldn't thin your paint out to the point that it just runs, this will give you little control. Normally, I thin Tamiya 50/50 and Model Master 40/60 paint to thinner. For Tamiya acrylics you can thin it down to 40/60 and drop the psi to 12-10 psi.
If you're still experiencing the air pushing your paint, you need to move back a little or decrease the air pressure.
Joel