What causes grittyness when airbrushing?

The paint is hitting the model surface partially dried.

Things to try:

• Reduce (thin) the paint more. 
• Get closer.
• Reduce the pressure.
• Combinations of the above. 

Thanks, Triarius.  Those are some of the things I was thinking also, but I wanted reassurance.  I didn't think about the air pressure, though.  I had raised my usual pressure a while back thinking that it would flow better especially for big jobs, perhaps that might be the culprit.  Getting closer might be the cause also, but that, and the way I thin my paint, hasn't really changed, just the additional air pressure mainly. 

Raising air pressure allows you to get better aspiration and atomization of thicker paint.

To get better flow, both through the brush and on the surface (leveling) thin the paint more. If for some reason you don't want to thin it (or you didn't get it thin enough) move in closer and drop the pressure slightly.

Paints that are intended for brushing with bristles are much too thick to airbrush—and most paints are manufactured to those viscosities because proper dispersal of the pigment requires them. Paint viscosity is adjusted after it has met color requirements.

As a general rule of paint-stained thumb, paint with "normal" viscosity has to be reduced 30 to 50 percent for aerosol application, whether from a spray can or an air brush. 

I've noticed a grittyness too, and I don't thin the paint at all. So I've noticed it kind of disappears fast, heh. How much would you actually thin the paint? I've tried once, and it seems to be too thin, so it doesn't leave a solid coat.

I have expereinced a similiar issue with Acrylic MM paint.  Grittyness or bumpy surface or finish.  I am assuming some of the same factors apply that the forum has listed.  However, I am not sure if Acrylic has some unique properties that would change your approach between Enamal vs. Acrylic.

My theroy was that since I was painting in my garage, and the temp was around 40 degrees the paint might be too cold causing it to be lumpy comming out of the airbrush.  Not sure if this makes sense. Any thoughts?

Adriaran: see the last paragraph of my post, above.

Unless you are very good with an air brush, I would never try to cover in a single coat. Several light coats are generally better. You can always apply another coat—getting the one the ran and sagged off, especially with enamels, is another matter.