Sunday, July 29, 2012

Tutorial: Airbrushing Basics

I'm going to follow the path of an air molecule moving through an airbrush system,  starting with the compressor, through the regulator(s), moisture trap, hose, and finally to the brush itself.


Canned Air


NO. Just no.

It's expensive in the long run, a source of inconsistently pressurized and unregulated airflow, and bad for the environment. Invest in a compressor.



Compressors - How I Tank?




A compressor is simply an engine that pumps the surrounding air into itself and expresses that air into a tank, regulator, hose, and finally airbrush. Compressors come in two major varieties - those made exclusively for airbrushing and the more widespread general purpose units (the picture above is a general purpose compressor). Either will do but there are some differences:


  • Noise: Purpose-made airbrush compressors are generally smaller and less noisy than GP compressors.


  • Size: Airbrush compressors are generally smaller and more compact than GP compressors. However, GP compressors will generally have a larger storage capacity (if tanked) and a stronger, more consistent engine.


  • Heat: Because of their size, airbrush compressors may not be able to handle long continuous spraying due to the buildup of heat within the engine. GP compressors generally do not suffer from this since they have some type of passive or active heat sink. This can be somewhat mitigated if the compressor has a tank (since the engine does not continually have to run to provide airflow).


  • Connections: GP compressors, being made to handle pneumatic tools and sprayguns, generally have larger pneumatic connections that may not fit an airbrush hose. This is easily solved with the installation of the correct fittings (available at any good hardware store) unto the compressor or regulator itself but the correct sizing may be difficult for novices.


I don't mention pressure above since that's really a function of the regulator(s) and, if you have it, the air tank. I personally own both types - a tankless airbrush compressor and a tanked GP compressor - but end up using the GP compressor more often simply due to its consistency and reliability.

On the subject of tanked vs. tank-less compressors, having used both, I always say spend the extra money for a tanked compressor. Tank-less compressors have one big disadvantage - inconsistency of air flow. With a regulated tank, I am always assured of having the same (or similar) pressure at all times, whereas even the best tank-less compressor will have fits and starts and have a higher or lower pressure behind the regulator.

Whatever you choose, it's important to purchase a new compressor from a trusted store or website. The compressor should be oil-less, have a grounded electrical cord, and be free of rust, chips, or dents. Remember shop safety - DO NOT PURCHASE A TANKED COMPRESSOR WITH ANY TYPE OF DAMAGE!


The Regulator(s) - "Mister, we deal in lead air."


Over the years I've become convinced that the regulator is probably the most important part in any airbrush system because it has the crucial job of maintaining the correct PSI in the airbrush system. A regulator is a valve that the user can adjust to "regulate" the pressure in the airbrush system. It operates using a unit of measurement called PSI, which is short for "pounds per square inch". PSI measures the pressure (or stress) of gases or liquids in a given system (The most widely known example being blood pressure, which ranges from 2.5 to 1.3psi in humans).


Without getting too in-detail, a regulator consists of three basic parts - an adjustment valve (sometimes a handle, knob, or nut) which either expands or contracts a diaphragm inside the regulator to force more or less pressurized air through the regulator. The pressure within the regulator is displayed on the pressure gauge.

Tanked compressors should have two regulators - one to measure and control the pressure within the air tank and another to measure and control the airflow to the airbrush itself.

Good regulators should be made of all-metal parts, have an easily controllable adjustment valve, and a large, clear pressure gauge.




The Moisture Trap  - Keeping it dry


A moisture trap (AKA moisture separator, condensation trap) consists of a filter which separates dirt and water particles from the compressed air after it leaves the regulator. There are two types of moisture traps - post-regulator and in-line.

A post-regulator moisture trap is screwed directly into the outlet port in the regulator. An in-line moisture trap is mounted "in-line" of the air hose, at least 6" from where the hose connects to the regulator. Due to the properties of compressed air (which I won't go into here), most professionals prefer in-line traps. Since these are usually more expensive (taking into account the hose cost) than post-regulator mounted models and most of you reading won't be using an airbrush on every single project, a post-regulator model should work quite well.

Whichever model you choose, be sure to clean and empty the trap of water periodically to ensure long-term use.



Hoses


Air hoses generally come in two types - coiled or straight - and two materials - PVC or braided nylon. I prefer a straight, braided nylon hose. Straight are generally lighter than coiled hoses and braided nylon is more durable and flexible than PVC.



The Airbrush


There are two categories (or systems) of airbrushes - double-action or single-action.
The diagram above shows the cross-section of a double-action airbrush. In both systems, compressed air enters from the air hose at the bottom center of the airbrush - this is the air valve. When the main lever is pressed down, the valve opens and air enters the airbrush. Here's where the difference in the two systems occurs.

In a single-action system, the air and paint are mixed inside the brush by pressing down on the lever. The spraying of paint and air is thus a single action, hence the name. When the main lever is depressed and pulled back, the needle slides back from the air nozzle and more paint/air are released.

In a double-action system, when the main level is depressed, only air enters the brush (and is consequently sprayed out). Paint is mixed into the air flow only after the level is pulled back - a double action (press for air, pull back for paint). This allows the user to keep air flowing through the brush, without having to spray any paint.

The next set of differences is how the paint "flows" into the brush itself. The diagram above is called a gravity-feed system. The paint cup is mounted atop the brush and paint flows into the brush via gravity. The other system is called siphon-feed. The paint cup is mounted below (or sometimes, to the side of) the airbrush and the compressed airflow "sucks" the paint inside the brush.

I personally recommend a double-action, gravity-feed airbrush for most modelers. The specific make and model is a personal choice (Paasche, Iwata, and Badger are generally the traditional, "go-to" airbrush companies) but you should look for the following features:


  1. All-metal construction, especially with moving parts
  2. Lightweight 
  3. Well-balanced
  4. Good access to replacement parts

Basic Setup & Painting


Be sure to spray in a well-ventilated area, away from small children and pets. Your painting area should also be out of direct sunlight and between 60-80F for optimal spraying. Unless you are airbrushing outside, you should be wearing, at least, an N95 Particulate Mask. Spraying completely indoors, even with a spray booth, means you should be wearing a full respirator.

To start, make sure all your fittings are properly connected - threaded couplings should be wrapped in teflon tape, firmly attached, and wrench tight. Make sure your airbrush is properly attached before turning on the compressor.

When the compressor comes on, let it run for a few minutes. If you have an air tank, let the tank come up to at least half pressure. After that - with no paint in the airbrush - push down on the main lever. You should see the gauge on your regulator come up - this is the current PSI you're spraying at. You can adjust the current PSI higher by turning left and loosening the valve knob (more air flow) and lower by turning right and tightening the valve knob (less air flow). For model work, you should be spraying somewhere between 20-35psi.

Add the paint to your paint pot and thin to the consistency of skim milk (see the section on Proper Thinning below). Keep a sheet of paper handy and spray out some paint unto the paper. If the paint doesn't spray well, thin a little bit more or adjust the PSI slightly higher. If the paint is thinned too much, it will run and drip. If this happens, add more paint and stir well.

With a double-action airbrush, practice letting your finger apply the right amount of pressure to the main lever. More pressure will increase airflow, and vice versa. When you feel confident, pull pack slowly on the lever to begin spraying the paint. As with the airflow; pulling back a little will spray a little paint, pulling back further will spray more.

You may also want to try some basic airbrush exercises to get your control correct.




Properly Thinning Paint

Let's make this simple - STOP THINNING ACRYLIC PAINT WITH FOREIGN CHEMICALS!!!


Windex, Ammonia, Flow Improver, that unmarked bottle of clear liquid from the medicine cabinet...whatever you've heard on such and such a forum on teh interwebs, these all cause more issues than what they solve and are generally more expensive per volume than dedicated acrylic medium.

I'd like to quote ScoutII over at TMP for the breakdown of why, chemically, thinning with foreign chemicals is a bad idea:

"Anywho, when you thin a paint like an acrylic paint with something like water (or Windex…alcohol…Flow Aid…) you are diluting the paint. When you use a formulated compound like an Acrylic Medium you are actually adding a different type of acrylic resin into the mix that has better physical characteristics for the task at hand.


This is important because when a monomer like the acrylic resin actually cures it forms a bunch of long chain polymers that are sort of ball shaped. They have all kinds of hooks and what not that let them snag other long chain polymers that are next to them and when you have a few million of these chunky long chain polymer spheres all hooked together they form a very strong…very durable film. It can bend and move without cracking and does it's job wonderfully.


When you dilute the compound with something like water, you put fewer of the monomer molecules together. When they start to polymerize into the long chains, there will be fewer of those long chains close to each other and it will reduce the strength of the resulting film. You can see this yourself if you dilute the paint significantly and paint it on a surface. The resulting film is almost dusty like and you can rub it off quite easily.


Now the actual Acrylic Mediums are basically just a different formulation of resin (with other chemicals added to adjust drying times and the like). The resin used though generally will flow like water (though it isn't water). This provides additional long chain polymers for everything to bind to when it is curing. You can use something like the Airbrush Mediums I linked to above to thin the paint to the point where you can barely see the pigment at all, and they will still produce a durable film.


I know, you will definitely hear from people who will swear up and down that they thin their acrylic paints with only water (or only Windex…or some other substance they find in their medicine cabinet) but it will result in a less durable finish."


There are numerous reliable options for Acrylic Medium (sometimes marketed as "Airbrush Medium") on the market from companies like Golden, Liquitex, Vallejo, and Testors; to name a few.



Cleaning


After each painting session you should thoroughly clean your airbrush, at the very least. Optimally, you should be doing a basic cleaning of the brush between paints.


To clean your airbrush:
  1. Empty any leftover paint from the paint pot
  2. Fill the paint pot with clean water and/or airbrush cleaner
  3. Run all the water/cleaner through the airbrush
  4. When the spray is clear, remove any leftover water/cleaner from the pot and blow air through the brush
  5. When no water comes out of the brush, turn off the compressor and remove the needle, according to your model's instruction manual
  6. Being careful not to bend the needle, wipe it clean and replace
  7. Clean any dirt or paint off the brush and thoroughly dry

1 comment:

Monty said...

Excellent. Thank you for this tutorial. I've been thinking of buying an airbrush but living in Minnesota, its winter 6 months of the year. I think I'd need one an air filtering devices to go with an airbrush.