Read the label to find out if the paint nozzle is a single or double action type. These two types of nozzles differ in their control of the paint and air. A single action nozzle uses one mechanism to control the air flow and a separate to control the paint flow. A double action nozzle can control the air and paint flow with one mechanism.
Ask for a number one nozzle if you are going to air brush something that takes small streams of paint and high precision, especially if your are using a low viscosity medium such as inks, water colors, dyes and stains.
Purchase a number three nozzle for virtually any airbrush-ready medium. This is the most popular grade of nozzle due to its versatility.
Use a number five nozzle for the thicker liquids such as glazes and high viscosity mediums, as well as the higher pigmented mediums such as latex and varnish.