1. Thin the paint properly - if your paint is too thin, it will run like water when you apply it. If your paint is too thick then it will either not come out of the airbrush or will come out in big globs.
2. Keep the proper distance from the model - if you paint too close to the model, you will end up putting too much paint on. This will cause a drip or run. If you are too far away, the paint will actually dry before it hits the model and will not adhere properly. The finish will look rough and will easily flake off the model.
3. Use the right pressure - your compressor should put out a nice even pressure. If your pressure varies you can end up having the same problems that your paint thinning has.
I will try and take some pics of proper
and put them on here. For now you can check out the
page and see the "Painting and Weathering Model Railroads"
It has an excellent explanation of airbrushing and its