I have found a thinner formula which is an improvement to my Vallejo Flow Improver mix using Vallejo Model Air.
This new formula is not mine, and thank the modeler (SpitfireNut22) on another site for it:
70% Vallejo Airbrush Flow Improver
30% Vallejo Airbrush Thinner
10 drops of retarder (I used Golden Retarder, original formula calls for Vallejo Retarder (which I don't have)).
I mix this in a Vallejo 35ml mixing bottle.
I mix this thinner formula 1:1, thinner to Vallejo Model Air, and I'm able to pull fine lines and mottling easily on plastic...minimal tip dry... you can complete the fuselage side of a 1/48 aircraft without any clogging or tip dry even in our arid and high altitude environment. My previous formula just using the Flow Improver and Vallejo Model Air is good but is a bit too translucent and is prone to spidering if not very careful.
I also find that it helps to spray acrylic paint, particularly for fine lines and mottling, on a flat painted base as one which is imparted by Vallejo Model Air or a flat primer - this helps preclude spidering and pooling with thinned paint. I always apply a gloss clear coat after painting in prep for decaling.