mixing paint

Depends what paint you are using. Most paint (acrylics and enamels) will have to be thinned. That's why I mix it in another cup before airbrushing.

ANdy

I'm going to assume that your question is asking "do you mix in a separate container" rather than "do you have to thin your paint" before putting in your airbrush.

I prefer to mix my paints in a steel measuring spoon (no particular reason for the spoon, other than it's available and convenient) before pouring it into my airbrush cup. I prefer to mix it "outside" the airbrush because paint consistencies vary from one jar to another and I like to see how it flows before it goes into my airbrush.

When I used to use a siphon feed brush, I mixed directly in the paint jar.

If indeed you are asking "do you thin your paint", the answer is yes. Most model paints are too thick to airbrush "straight".  Even though there are some which can be airbrushed without thinning (eg. Gunze acrylics, Vallejo "Model Air") these paints do benefit (smoother finish) from a little thinning.

I mix in a seperate container for a couple of reasons - first I find that I can get a better consistency of paint (skim milk???) that way and secondly, I can strain the thinned paint to remove any possible lumps (that's a carry over from the days I painted 1 to 1 aircraft and cars on a semi-regular basis - theory is still valid for an airbrush though - you haven't lived until you get about 1/4 to 1/2 done on a painting project and the paint flow quits because you've sucked up a lump of unmixed or dried paint!)