Airbrushing for Modelers 101

Today, almost all serious modelers would agree that an airbrush is essential.  However, it has not always been that way.  The first airbrush was actually invented in 1876 by Frances Edgar Stanley, who is best known for the Stanley Steamer.  It was basically an atomizer with the addition of an adjustment for material flow.  In 1879, Abner Peeler invented, what many consider to be the first real airbrush.  Known as the “Paint Distributor,” it had an external turbine and functioned much like the modern day Paasche AB.  The first double action airbrush was patented by Charles Burdick in 1890 and produced by Thayer & Chandler in 1892.  His design is the basis for most modern airbrushes.  Despite this long history, it was not that long ago that modelers debated the necessity of what, to some, was deemed a luxury.  An argument I can well remember having with my father as I took my hard earned money downtown to our local art supply store to purchase my first airbrush.  It was a jewel-like Paasche model F in a velvet lined case.  Some 45 years and countless airbrushes later, there is still a uniquely special thrill with the purchase of a new airbrush.

History of the Airbrush
History of the Airbrush

In design, the airbrush if very simple.  It uses the function of Bernoulli’s Principle (a pressure differential) to draw the material into an airstream and through a venturi effect, the material is accelerated and atomized.  Where the material and air are combined differentiates two major categories of airbrushes: external mix or internal mix.

External mix airbrushes, as the name implies, combine the air and material outside of the airbrush, or “externally.”  The classic external mix airbrush is the Paasche Model H.  The adjustable fluid tip is located just ahead of the orifice in the air cap.  The fast moving air passing over the fluid tip siphons the fluid into the air stream where it is atomized and propelled.  The adjustable fluid tip is quite simple and is comprised of only two basic parts:  a hollow needle through which the paint is drawn and a threaded tip that allows the adjustment of the material flow.  A rudimentary adjustment screw is sometimes provided under the air button, but for the most part airflow is basically on or off; controlled by what is essentially a tire valve.

single action airbrush diagram

Internal mix airbrushes combine the air and material much closer to the airbrush.  The term “internal mix” is a bit of a misnomer because it implies that the material and air are combined within the airbrush, but that is actually not the case.  With this type of airbrush, a centrally located needle and tip regulate the material flow, while the head assembly channels air around the outside of the tip.  The orifice of the material tip is usually located flush with the front surface of the air cap.  So the air and material are combined just as they both exit the airbrush.  As with the external mix airbrush, the airflow is essentially on or off.

double action airbrush diagram

This diagram represents the head assembly of a simple internal mix airbrush such as the Badger 150.  Material is introduced and flows along the needle within the passageway formed in the head and tip.  As the needle is drawn back, material flow is increased.  Air is channeled forward and around the tip.  As the air moves forward, it accelerates because the size of the passageway is decreasing.  The fast moving air draws the paint along the tip.  Complete atomization occurs at the tip of the needle as the paint is directed off.  So dispite the name “internal mix” complete atomization occurs at the needle tip, which in many cases is external to the material tip.  The atomization is more complete than the external mix airbrush because the air is introduced in a tight, forced, circumferential flow around the material tip.

Depending on the design of the airbrush, the forward most piece may function differently.  This diagram is of a simple internal mix design.  In this case, the front-most piece is integral to the airflow.  Because of this, it is labeled as an air cap.  Loosening this piece slightly will disturb the airflow and cause stippling.  Loosening it more will cause air to flow back through the material tip.  More sophisticated designs may use a separate air cap and paint cap or crown cap.  A paint cap is used to control the pattern or spread of the spray.  The function of a crown cap is to protect the needle.  Removing a crown cap does not disturb the airbrush function and allows the artist to get closer to the surface.  Without the crown cap in place, care must be exercised so that the needle is not damaged.

internal mix airbrush head design airbrushing for modelers 101

Airbrushes are further distinguished by what is termed, their “action.”  The “action” refers to how the material flow is regulated.  Airbrushes are either single or double action.  Single action airbrushes separate the control of the material and air.  The airflow is controlled with the trigger, by the hand that is holding the airbrush, and the material flow is adjusted with the other hand.  Two examples of single action airbrushes are the Paasche Model H and the Badger 200, Figs. 7, 8.  While most single action airbrushes are external mix, the Badger 200 is unique in that it is internal mix.

Paasche Model H
Badger 200

Double action airbrushes allow the adjustment of the material with the same trigger that controls the air.  This provides much more precision and the ability to vary material flow while spraying.  After depressing the trigger to begin the airflow, the trigger is rocked back to allow the flow of material, Fig. 9.  Double action brushes can be outfitted with a preset handle that will allow the setting of maximum desired opening, Fig. 10.

Badger 150
Iwata HP-BC2

There are two main ways that material is fed into the airbrush.  The first method is known as gravity feed.  In this style of airbrush, the material cup is located most commonly on top of the airbrush, Fig. 11.  However, some airbrushes are fitted with a side cup.  The side cup still functions as a gravity feed while providing the freedom to alter the angle of the cup.  Here again, the term “gravity feed” is deceiving.  The material is still siphoned from the cup; otherwise, the material would be flowing all the time.  So, in gravity feed airbrushes, gravity is only offering assistance to the material flow.  This type of airbrush works better for thinner materials where the air pressure can be reduced.  Lower air pressure means a tighter pattern so you can work in close with more accuracy and sharper lines.

The other method for delivering the material is siphon feed.  In this style of airbrush the material cup is usually mounted beneath the airbrush and relies solely on siphon effect to move the material, Fig. 12.  Although not as efficient, some gravity feed airbrushes with side mounted cups offer a siphon feed style bottle as well.  Siphon feed is particularly suited for moving larger volumes of heavier material such as paint.  The siphon cups are a major advantage for color changes and volume of material.  These airbrushes operate best on higher air pressure.

A note about side cups: As a further explanation, side cups can actually be gravity or siphon type depending on whether the paint in the cup is above or below the centerline, or needle, of the airbrush.  Or to put it another way, if the side cup can function with an open top, then it is gravity fed.  If on the other hand, the lid of the paint cup has a siphon tube in it, then it is obviously a siphon fed cup.  Siphon feed side cups are not particularly common but it is worth mentioning in order to add more confusion.

Harder Steenbeck Infinity
Paasche VL

Finally, there are a couple of other airbrush design elements worth mentioning.  Some airbrushes are equipped with what is called a MAC valve, Fig. 13.  The MAC valve is an attempt at providing air pressure regulation at the airbrush.  This can be of value to illustrators who find it helpful to have quick access to air pressure regulation.  I’ve talked with very few modelers who find it useful.  In this authors opinion it is a questionable feature for model painting since we are not constantly varying air pressure which can be more precisely managed with a regulator.

Rather than using a button, some airbrushes use a trigger, Fig. 14.  Trigger operation is a bit different.  The feel is more like that of a spray gun.  When the trigger is first pulled, airflow begins.  As the trigger is advanced, the material needle is drawn back and the material flow increases.  If you haven’t used an airbrush with this configuration, you might want to give it a try.

At the beginning of this page, there is a photo of a unique style airbrush.  For some, it is the Holy Grail of airbrushes.  It is the legendary Paasche AB.  This particular style of airbrush is known as a turbine airbrush.  The turbine is used to coax the material into the air stream much like the Peeler Paint Distributor.  It is designed for use with inks, dyes, and watercolors.  I only mention it here for those who are curious.  It is not suitable for use with paint.