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Single-action vs. Double -action
Siphon-feed vs. Gravity-feed
Needles and Nozzles
Controlling Atomization
Matching to the Proper Pressure

Types of Airbrushes

Single-Action Airbrushes vs. Double-Action Airbrushes

"Single-action" and "double-action" refers to how the artist controls the airbrush’s air and paint flow. Single-action means that it sprays much like an aerosol can: just push down the trigger to get it to spray. The amount of paint that comes out is controlled by twisting a knob or screw located near the tip. This type of airbrush is also referred to as an "external-mix" because the air and paint actually mix in front of the needle. There are fewer moving parts that need to be cleaned, therefore it is an easy airbrush to maintain.

The trade-off is that one does not have the precise control offered by a double-action. However, a single action is a great beginner’s brush that will always have uses no matter how advanced an artist becomes. Smooth gradations are easily accomplished, and for many artists, this will be the only airbrush they will ever need.

For the best control, such as fine lines and the ability to produce thick-thin strokes (the classic "dagger stroke"), a double-action airbrush is essential. Also known as "internal-mix" airbrushes because the air and paint mix inside the nozzle, this type of airbrush has two trigger movements. As with the single-action, air is controlled by pressing the trigger down.

The amount of paint spraying out is controlled by pulling back on the trigger. The farther the trigger is pulled back, the more paint comes out. This allows for absolute control of the paint. T-shirt lettering is vastly easier with this type of airbrush. Experienced, "freehand" (painting without the aid of masking materials) airbrushers can control a double-action to produce photo-realistic artwork.

Siphon-Feed Airbrushes vs. Gravity-Feed Airbrushes

These terms refer to how the paint is supplied to an airbrush. Although this feature does not directly influence how an airbrush performs, it does indicate the paint capacity allowed before refilling.

Siphon-feed means that the color-cup attaches from underneath the body of the airbrush. Air suction pulls the paint from the cup to the nozzle area, where it comes in contact with the air. This type is useful when spraying for extended periods of time because the color-cup (typically 1/2 ounce capacity) can be taken off and a bottle can be attached, normally with a capacity of 3 fluid ounces or less.

Gravity-feed essentially means that the color-cup is on top of the airbrush body. Most models have an immovable color-cup. Although larger models can have paint reservoirs of 2 ounces, gravity feed airbrushes are made for detail, where small amounts of paint are applied at one time. Because the paint is in an open color-cup, some models offer a separate cap to keep paint from drying out in the color-cup. They have a tiny hole in the center of the cap to prevent a vacuum from developing. It is essential that this hole be open to maintain proper paint flow. Siphon-feed bottles also have this hole on the cap for the same purpose.

Needles and Nozzles

The Airbrush Needle and Nozzle

All airbrushes work on the same principle; air and paint meet at an exact point in space. At that point in space there is a tapered "needle" that projects the combined air and paint forward. This simple concept was used by Neolithic man on the walls of his cave. He would orally grind up the "pigments", anything from charcoal to berry juice, in his mouth, with saliva being the "binder". His lungs were the air source, his mouth was the nozzle and his tongue was the needle. Fortunately, the airbrush was invented around the turn of this century, making the modern airbrusher’s life much easier.

Obviously, a human tongue is a very blunt, tapered needle that will not allow an artist to spray fine lines. Fine line spraying requires very delicate needles tapered to a delicate point. Large commercial spray guns, such as those used for automotive painting, have blunt needles that do not have much of a taper. However, their shape is made to fit into a large nozzle that can deliver great amounts of paint. The finest detail airbrushes have a nozzle size of .18mm.

The "nozzle" is the part of the airbrush head assembly in which the needle rests. It is tapered exactly the same as the needle it holds. On a single-action airbrush, when the screw is twisted, it widens the space between the needle and nozzle by moving the nozzle. In a double-action airbrush, when the trigger is pulled back, it moves the needle back as well. The farther back the needle moves, the larger the space between the needle and nozzle.

Controlling Atomization

What is "Atomization" ?

Atomization refers to how fine the particles being sprayed are being broken apart. In a garden hose, when there is a light mist of water sprayed, the nozzle is finely atomizing the water. Higher water pressure will allow for finer atomization. In an airbrush, instead of water pressure dictating the atomization, it is accomplished with air. Airbrush nozzle head assemblies have amazing baffling systems that funnel the air around the needle so that the air can atomize the paint evenly. This action gives the airbrush precision.

The better the atomization the smaller the drops of paint and the closer together they are. This provides a more solid coverage and finer lines.
Atomization in airbrushes is the same as DPI (dots per inch) in computer printers. The more dots per inch the higher the resolution, which results in better output. In airbrushes the higher the atomization the crisper the image. Atomization will be rated on a 1-10 scale (1 being poor and 10 high) for a better idea of the spray quality of the brush.

Atomization is Controlled by Air Pressure, Air Baffling and Paint Thickness

While there is nothing that one can do to alter the baffling of a given airbrush, the air pressure and paint thickness, or "viscosity", can be adjusted to suit a particular need.

Air pressure is measured in pounds per square inch, commonly referred to as "P.S.I.". As mentioned above, pressure influences atomization. In an airbrush equipped with a fine needle, less pressure is required to produce good atomization. This important aspect needs to be addressed before one can decide what the proper paint viscosity is.

Not enough pressure results in a stipple paint effect, too much pressure causes "overspray", a soft halo of color surrounding the area intended to be sprayed.

Paint Thickness Differences

The viscosity of the paint to be sprayed is also equally important. Viscosity is measured in Centipoise (cPs). 1 Centipoise is the resistance of water, hence water has a viscosity of 1 cPs. Golden’s ready to spray Airbrush Colors have a viscosity range of 40 - 60 cPs, making them ideal for illustration and fine art. Most textile airbrush colors range from 100 to 400 cPs. A typical house paint is 3000 - 6000 cPs.

If the airbrush is set to an adequate pressure and spattering still occurs, the paint is too thick to be properly atomized. Sometimes raising the P.S.I. can eliminate the spattering, but the correct procedure is to thin the paint. Over thinning can also have adverse effects, therefore switching to a larger needle/nozzle airbrush is also an option the artist must consider.

Matching an Airbrush to the Proper Pressure & Paint System

By understanding all of the above information, an artist can choose the type of airbrush needed for each specific task.
For example, when excellent atomization and precision spraying is required (such as with commercial illustration), an airbrush needs to have a fine needle that will work with low pressure (15 - 40 P.S.I.) and low-viscosity paints.
Conversely, t-shirt artists prefer a thicker paint because it will have less overspray. They tend to spray at 60 - 100 P.S.I. and require an airbrush that will accommodate the higher pressure and thicker paint.
To properly spray housepaint, the spray equipment has a large nozzle and a different baffling system that will allow it to spray at 40 - 60 P.S.I. but atomize like it was at a higher pressure.