air pressure

You've come to the right place for answers.  But first, to better help you, answer these questions:

1. What make of airbrush are you using
2. What tip are you using for your airbrush
3. What air source
4. What paint

-Jesse

Lacking any specific info, 15 PSI should at least be a decent starting point for air pressure.  I have a decent variety of airbrushes and all will paint fine at that air pressure.  From there experiment with things.  Your variables are the air pressure, the paint thinness, distance of the airbrush from the model, and your needle position and any can affect your painting.

For fine lines, lower airpressure will usually better control, but if you lower it too much, the paint will not atomize as well.  When you're painting fine lines, you need to get your airbrush tip close to the model, maybe on the order of 1/4" - 1/2" distance. 

As for thick paint,its best avoided.  If your paint is so thick that you have to crank up the pressure way beyond that, you probably need to thin your paint some more.  If your paint is too thick, you won't get nearly as nice a finish as you could with properly thinned paint.

As Jesse said, if you can supply some more specific information, we can give you more informed suggestions.

Andy

shivinigh wrote:

I have a pashe H single action airbrush with an Iwata compressor. I am using the smallest tip I have with Model master acrylic paints.

Paasche H is one my airbrushes.  I use it for all my acrylic painting and typically paint with it at 15 PSI.  To give you an idea of what the brush should be capable of delivering, here's a photo of a 1/72 JagdPanther that I painted with mine.  The weathering and dirt obscure it a bit, but the green lines on the side skirts show through pretty well.

I painted it with Tamiya acrylics which paint the same as ModelMaster acryls.  I used the small No.1 tip which is identified by having a single engraved line circling the tip and air cap.  The medium No.3 and large No.5 tips have three and five engraved lines respectively.  I probably painted this at 15 PSI, with the tip of the air brush probably 1/4" or so from the model.  The tip would have been adjusted to be mostly closed.  Under such circumstances, the paint needs to be perhaps a bit thinner than what you might want for laying down a base coat.

Andy

I appreciate the help. Time to go practice some small line airbrushing

I got a Paasche H also.  I've tried to use Model Master Acryl paints for fine line painting with my Paasche, but I've never had good success.  To me, the paint doesn't seem to atomized well with an external mix airbrush.  Now I have had good results with Tamiya acrylic paints.  I too have done free hand camo work with my Paasche H and Tamiya paints.  You might want to consider getting Tamiya paints or using Model Master enamels.  If you plan on using the Acyl paints, I would recommend using their thinner for thinning the paint.  I would avoid using water or alcohol.

-Jesse

15-25 psi is best for most situations.

Other than that, the best advise is "practice practice practice"