Paint and water don't mix well. If the paint is water-based, it gets diluted, if it's not the colors won't register and stick. Water getting into the airbrush has long been a problem of airbrush users. Theories and their corresponding solutions have been advanced, some junked, others used with reservation. The problem has been the subject of numerous blogs on airbrushing websites and forums. Gadgets and devices have been invented to minimize or solve the problem altogether and have met with mixed reception from users.
The problem appears to come from condensation in the compression chamber of the compressor, if it has one. Condensation occurs when air or gas is compressed in a sealed chamber, in this case the compressor's chamber or air tank and there is a change in temperature. The particles of water are ejected together with the compressed air into the airbrush, where it mixes with the paint and affects the flow of paint as well as the color results. Most compressors with compression chambers are equipped with air valves to purge the water in the tank. However, this sometimes does not do the job thoroughly although purging the air tank frequently helps.
The common solution employed today is to have a moisture trap with a filter to prevent the water condensate and any other foreign particles from entering the airbrush. If the compressor does not have a trap and filter, then one needs to be installed. The trap and filter may be located at the top of the compressor where it connects to the airbrush, or inline somewhere along the hose that carries the compressed air to the airbrush.
When the airbrush starts to spit out water, the recommended solution is to first detach the trap and filter from the air hose. The filter should be examined for defects and replaced if necessary. After replacing the filter, the trap should be reattached.
Another gadget that is offered as a solution to this problem is a small anti-water filter. Another form of a filter and trap, it is placed at the mouth of the air tube and attached directly to the airbrush. The device is made of transparent plastic and aluminum. The transparent plastic enables the user to see if water has accumulated in the device so it can then be removed through the small purge valve of the device. The aluminum part of the device is to make it more durable.
Another suggestion to solve the problem of condensate is to attach an external fan to the body of the compressor. This is meant to eliminate temperature changes which cause the condensation. Those who have tried this method report that it works, but only for a brief period.
Still another suggestion is to make sure that the compressor has an automatic stop feature which turns the compressor motor off when the pressure reaches 4 BAR and restarts it when the pressure drops below 3 BAR. This delivers pressure of 45 psi when the motor is on and 60 psi when it is off. Automatically shutting the motor off and on prevents the compressor from getting too hot and causing the condensation.