Make sure the cake decorating airbrush has never handled non-edible or toxic paints. If an airbrush is not thoroughly cleaned, it can carry paint residue from its previous use.
Set the air compressor to charge to the appropriate PSI (Pounds per Square Inch) as designated by the manufacturer's instructions. Connect the airbrush to the air compressor.
Press down on the airbrush trigger to make sure the air comes out. If the air does not come out, check the hose to make sure the airbrush is correctly connected to the compressor. If the air still does not come out, remove the airbrush from the air compressor. Take apart the airbrush trigger mechanism, following the manufacturer's instructions. Some trigger mechanisms can jam or be difficult to install correctly. Put together the trigger mechanism following the manufacture's instructions.
Attach a canister of edible paint to the airbrush. Hold the airbrush's nozzle between 6 and 8 inches from the sample cake's surface, at a 45 degree angle. Press down and back on the trigger to release paint. If there are bubbles in the paint, or intermittent paint spraying, this means that air is leaking into the paint. Rub beeswax on the portion of the airbrush between the nozzle and body to prevent any air leakage. If only air comes out, tighten the needle lock nut. If the color is intermittent, examine the needle to make sure it isn't bent. If the needle is bent you will have to purchase a new one and replace it.