Have you ever tried using spray paint without shaking the can up first? It comes out very uneven. That's because the paint is not a uniform thickness. If you are using the paint in an airbrush without thinning it, you can get much the same result as using spray paint without shaking it up. As batninja stated, you can usually use water to thin acrylic paints, and it will work just fine in an airbrush. The thinner just lets the mixture stay a uniform thickness longer, and will not start to separate as fast. If you're painting alot of one color, you may want to consider using the thinners.