When I asked what size needle I meant the tip size , not the needle shaft diameter .
It will be in the range of .2 to .6 mm . ( like .25mm , .35mm , .65 mm etc - depends on the brand of airbrush )
Does your airbrush have only one size spray tip and corresponding needle ?
To recap ,What paint and thinner ?
and how much thinner are you using ?
20 psi should be enough for most coatings -- is that 40 psi the tank or compressor discharge pressure , before the regulator ?