Jim Ryan
One of the most useful finishing tools the scale modeler can own is a good airbrush. While some modelers (especially giant scale enthusiasts) prefer automotive detail guns for painting their creations, nothing else quite matches the pin-point control and convenience of a good quality airbrush. A good airbrush is light, easy to handle, and quick to clean up.
1. Types of Brushes:
There are two general ways of grouping airbrushes into classes: They're either external or internal mix, and the paint metering is either single- or double-action. Here's what these terms mean:
External Mix: Liquid paint is introduced into the compressed air stream outside the tip of the brush. In general, this type of brush is considered faster to clean, but it requires careful adjustment and doesn't tend to produce as fine a pattern as an internal-mix brush.
Internal Mix: Paint is mixed with air in an internal plenum in the brush. Because of the multiple air jets and the internal mixing, the paint is usually more fully atomized, resulting in a finer, tighter spray pattern. I also find this type of brush easier to assemble, in that it's essentially self-adjusting.
Single-Action: The air valve is simply depressed to release air, and the resulting venturi effect at the tip draws paint from the mixing cup or jar. Adjusting the paint flow, if possible at all, has to be set between shots.
Double-Action: On a double-action airbrush, depressing the air valve releases air only, and the brush has a separate internal needle valve for controlling paint flow. To operate the brush, you depress the button to start air flowing, then you rock the button backward to release paint into the air stream. While it takes a little practice to get used to, this allows you to constantly adjust the paint flow to suit your needs. Believe me, this micro control is a tremendous asset.
Generally speaking (there are exceptions to every rule), the
cheapest
models are single-action/external mix, and the best units are
double-action/internal
mix. Photos 1 and 2 show examples of both types.
2. Air Sources:
Of course, you can't use an airbrush without air, so I'd like to take a moment to touch on some compressed air sources: One of the simplest and cheapest air sources is a large spare tire, with an appropriate valve to mate with your airbrush's air line. A similar option is a compressed air bubble. The weakness of both these options is that you have to visit a gas station to re-fill them, and that can get a little old after the 6th or 7th trip.
A more compact choice is aerosol cans of ozone-safe propellant. These have the virtues of being compact and quiet, but you once again have the problem of running out of propellant right in the middle of a painting session. Besides, by the time you've purchased half a dozen cans of propellant, you could have paid for a good compressor.
When most people think of air compressors, a piston compressor with air tank comes to mind. This type of compressor works very well, but the clattering racket it produces can annoy family members.
My personal preference is a diaphragm compressor. In operation, diaphragm compressors are similar to aquarium pumps, and like their smaller brothers, they're very quiet. A first-rate unit that will last indefinitely can be had for around $100. Frankly, if you've been running back and forth to the gas station to re-fill your tire or air bubble or shouting over the noise of your piston compressor, the first time you use a diaphragm compressor, you'll wonder why you waited to buy one.
I'm fully aware that some of you are already using modified Freon or
propane tanks for compressed air tanks. I do not recommend this. First
of all, these light-duty tanks are made from stamped sheet metal and
are
not designed for repeated high pressure use. Second, with no water trap
or interior coating, they're prone to corrosion that will further
weaken
them over time. With all the other options, this just isn't a good
trade-off.
3. Masking Materials and Techniques:
Even with the pinpoint control of an airbrush, there are going to be times when you need masking of some kind. I'd like to loosely group masks into two classes; those that provide a soft edge and those that provide a hard edge.
Soft-edge Masks: Some of the color boundaries on a scale model are more defined than even an airbrush can provide, and yet they aren't hard enough for traditional masking. An example is the jagged splinter camo on the upper surfaces of WWII Luftwaffe aircraft. Also, the model may need a feathered line, but it might be longer and straighter than your wandering hands are capable of replicating reliably. A typical example of this is the three-tone camo seen on mid-WWII US Navy aircraft.
In these cases, I make a mask out of posterboard, sheet styrene or other material. This mask can either be laid directly on the surface for a fairly hard edge, or held farther from the surface for a progressively softer effect.
Hard-edge Masks: There are a lot of options for masking hard edges. Good old masking tape has its adherents (no pun intended), but in my opinion, there are better choices. If you do want to use paper masking tape, I recommend 3M 2090, a lower tack professional grade tape that's blue in color. This low-tack property is very desirable if you've ever peeled off the paint along with the tape. My personal choice for masking tape is 3M 218, a vinyl striping tape. This material has low-tack, excellent flexibility for masking curved lines, and provides a razor-sharp edge with almost no chance of bleeds.
For masking numbers, insignias and other graphics, I like to use artist's frisket. This sheet material can have the graphics copied onto it with a photocopier, and it can either be pre-cut before application or even cut to shape on the airframe. Note that some aggressive paints like acrylic lacquer may attack the adhesive in the frisket; it's always a good idea to test the paint and mask for compatibility. My article in the February, '97 issue of Model Airplane News covers the use of frisket in more detail.
Before departing the subject of masking materials, I'd like to touch
on a handy product that can be found at your local paint store. It's
variously
called "Ready Mask" or "Easy Mask", and it's like a roll of "Post-It"
note
material. The masking material is thin brown paper with a low-tack
adhesive
along one edge. While I don't use it for the actual masked edge, it's
terrific
for quickly masking off surrounding areas from overspray.
4. Types of Paints:
It's beyond the scope of this article to consider each and every paint available to the R/C modeler, but a few general guidelines are in order. Because of their internal construction, airbrushes are best used with highly fluid paints that thin readily. This means that inherently thick finishes like the latex paint favored by some giant-scalers may not be suitable. Nearly all paints need to be thinned to some extent for optimum spraying results, and I typically thin my paints anywhere from 25 to 50%, depending on the paint and the specific task. Generally, you want the paint to be thin enough so that a test chip draws out smoothly in a few seconds. If it's thick enough that it retains a pebbly surface, you either need to add more thinner or vary your spraying technique.
Personally, I like the spraying characteristics of enamel and urethane paints. They can be thinned with mild mineral spirits and stay wet long enough to draw out smoothly. Then they can be dried quickly with a heat gun in preparation for the next coat. Clean-up is simple and the fumes aren't too objectionable. Because of the mild thinner, they work well with masking materials of all kinds. On the down side, some of these paints aren't fuel-proof, so if you're going to be flying with glow or gas power, a fuel-proof clear coat may have to be applied.
Other popular hobby finishes suitable for airbrushing include butyrate dope and epoxy finishes. When I flew primarily glow power, I really liked epoxy paint, and I've gotten excellent results by spraying it highly thinned. Of course, dope and epoxy both use rather aggressive thinners and produce dangerous fumes.
Another useful option is automotive-grade paints like acrylic lacquers. Be aware that some automotive paints have extremely toxic fumes, and even a regular respirator is insufficient protection. In these cases, a positive pressure breathing apparatus is required.
A promising area of paint development is the new water-borne paints coming on the market. These include water-based urethanes that cure to become fully fuel-proof. I have no experience with these, but they're an interesting option. Water-based acrylics have a longer track record, but some of these lack the toughness desirable for an R/C model, and nearly all require a clear-coat.
Whatever paint you choose to use, be sure to read the label and
observe
all safety precautions. Even some water-based paints can produce
potentially
dangerous fumes, and you should always wear an approved respirator and
use adequate ventilation.
5. Painting Technique:
Technique is everything when painting a model aircraft. Because paint is both heavy and expensive, you want to use as little as possible to get a good finish. One of the most common mistakes I see inexperienced painters make with an airbrush is that they hold the brush too far from the surface. This not only wastes paint due to excessive overspray, it also tends to produce a gritty surface with poor adhesion because the paint is partially dry before it even hits the airframe. The proper technique varies from one type of paint to another, but in general, I like to stay about 6" from the surface. Work on producing nice uniform overlapping strokes that look smooth and wet. If the paint isn't wetting out, it won't draw out smoothly, and the surface will have an uneven chalky appearance when you're finished. While most people keep the brush farther from the surface for fear of getting runs, it seems that the opposite is more often the problem. For fine detail work like painting the speckles on the side of a Luftwaffe aircraft, I frequently work with the tip less than an inch from the work surface. As long as you carefully meter the paint flow, runs won't be a problem.