Practice
A common question that shows up in modeling forums goes something like:
"Why is my paint job so rough?"
I had the same problem when I first started using my airbrush. The answer is that the paint is partially drying before it hits the surface, and the finish looks dusty. The airbrush atomizes the paint so fine that it drys very fast Another problem you might see is orange peel. You can see an example and solutions here. This happens if the droplets dry before they have time to flow together. And, of course, you can get runs if you put too much paint on too fast.
Finish quality is affected by several variables, including paint consistency, air pressure, distance from the surface, and speed of brush movement. Maybe you are one of those people to whom airbrush use comes naturally, and you can produce great results right off. I am not one of those. It took me a while to get the feel of it. In the process, I got pretty frustrated. It's especially frustrating if you're trying to paint something that matters, like a model that you have spent a lot of time on. And, it's very easy at this point to blame the brush. But there was nothing wrong with the airbrush, I just had to learn to use it.
New Airbrush?
So you got a new airbrush. What's the first thing you should do? Well, as strange as it may sound, I think you should take it apart. Eventually you'll probably need to do it anyway, so you might as well learn how while it's nice and clean. And, you will have a better idea of how it works.
Don't do anything that the manufacturer says will void the warranty. For the Aztek, this pretty much means don't do anything.
The manual might have instructions, and it should at least have a parts breakdown diagram. If you do a search on YouTube, you may find a video on your brush. Sit down at a well lit workbench or table and spread a cloth or something so the little parts don't roll away. A dish or cup to hold the parts wouldn't be a bad idea. Pay attention to how the pieces are oriented. I wouldn't dismantle the air valve assembly. It is especially delicate, and rarely needs any maintenance. Also, you probably shouldn't mess with the needle bearing.
One of the first things I would do is carefully pull the needle so it won't get damaged. If there are any knurled parts that are too tight to loosen with your fingers, try not to use pliers. You'll probably ding up the chrome. Instead, try a plastic or wooden clothespin. Usually this will work. If it doesn't, protect the chrome with something like a piece of leather. A nylon Tywrap as shown on the right works well to protect the finish.
Note: A trick I learned from Ken Schlotfeldt, is to use the plastic head protector that comes with some Badger airbrushes as a grip.
Take your time and be careful and you should do all right. Don't install the needle until after the head is assembled, and then, hold it loosely when you insert it and make sure that the trigger is all the way down when you slide it in. You should feel a little friction going through the needle seal, and then a hard stop when it seats in the nozzle. Don't press too hard.
Try it out
Before you try expensive model paint, I recommend something cheap like ink or food coloring. Being water thin, you don't have to worry about thinning, and there won't be any smelly toxic fumes either. Prop up a piece of cardboard for an easel, clip on a paper towel and go at it. Play with it. Draw circles or boxes, or write your name. Get a feel for the brush. There may be exercises in your manual. Do them. Practice until it comes as second nature to press down for air, pull back for spray, let the trigger go forward to stop the spray, and then release the trigger to stop the air. It should always be: air on, paint on, paint off, air off. This will reduce splatters and make for a happier painter. I've read that some people have modified their brushes so that the air is on all the time. You can do this by just removing the air valve spring. But, to me that seems a little Mickey Mouse. One common exercise is to do dots, lines and daggers.
When you feel like you're ready for some real paint, practice on something disposable, like plastic bottles. They are free and they take paint just like a kit. Buy some of the paint you plan on using and experiment with all the variables until you have a feel for the brush and can consistently produce a nice smooth coat. It will be money and time well spent. I wish I had done this in the beginning. I still keep a couple of bottles handy and give them a couple of shots before spraying a model. And, if I have any paint left over in the cup, I use it for practice on a bottle. See samples on the right. I also use them any time I try something new. For instance, decals vary quite a bit in their tolerance to decal setting solutions. So I always find something on a sheet that I don't need on the model and try it on a bottle first. That way I know what to expect when I do the real thing.Some people will advise that the best way to learn to airbrush is to jump right in with real paint and a real model. I tried that and it didn't work very well for me. As the saying goes, your mileage may vary.Thinning I should say something about thinning. The instructions on a bottle of Testors ModelMaster Airbrush Thinner say to thin 3 parts paint to 2 parts thinner. I read a thread on the Britmodeler forum here where a fellow who used to make a living painting cars wrote that paint should be thinned as much as 1 part paint to 9 parts thinner. The most common advice is to thin to the consistency of 2% milk. I think the truth is that there is a wide range of thinning that will work depending on your technique. Fine nozzles, like .35mm and under, are going to require more thinning and produce thinner coats than the larger sizes. In general, thicker paint requires more air pressure to spray well.
One suggestion I have seen is to actually put some 2% milk in your airbrush cup. Tilt the cup and see how it coats the side. That's what you want your paint to look like. Be sure and clean up with some warm soapy water, though.
Another is to dip a stirring stick in some 2% milk and see how it drips off.
A lot of people thin Model Master enamel about 50/50 with lacquer thinner, and it has always worked for me. If you're going for really fine lines or your brush has a really small nozzle, you may have to thin more. You are the only one who can determine what works best for you. I use regular hardware store lacquer thinner with no problem. But, I have read that lacquer thinner can contain a variety of solvents, so I can't guarantee that they will all perform as well. Some people use mineral spirits with enamels, both for painting and cleanup. The only drawback I can see is that it would take a bit longer to dry. But, maybe that could be a good thing, and the fumes wouldn't be as bad. Testors airbrush thinner is about 50% mineral spirits.
A common question on forums is: "What should I use to thin Brand X paint?" or "Is it OK to use Liquid Z to thin Brand X paint?". Of course it's always safest to use the paint manufacturer's own thinner. But that can be a bit expensive, and people like to find cheaper alternatives when they can.
Well, don't do what I did. I was experimenting with tempera paint, and I poured a little alcohol in the airbrush cup to see how it would work as a thinner. As I stirred it, I noticed little clumps of pigment forming. So I figured I'd just dump it out and flush the brush with some Windex. It didn't work. Lacquer thinner? Nope. Acetone? Nope. In the end, I had to physically swab every nook and cranny to get it out. One of the guys on a modeling forum introduced me to a new term; flocculation. It can happen when a paint and thinner don't agree.
But it's really pretty easy to find out for yourself if something is going to work. Just put a couple of drops of the paint in something like a small saucer. Then add a couple of drops of the thinner you want to try. Stir it around. Does it mix well, or clump up? If it looks OK, let it dry. Does it dry OK in a reasonable time and does it harden up OK? If it passes this test, try some in your airbrush and see how it sprays. But remember, whenever you are trying something new, like a different thinner, it's safer to try it on junk plastic first.
Air Pressure
What about air pressure? There is no correct air pressure. It depends on your airbrush, nozzle size, paint consistency, and you. It could be as low as 10 psi for thin paint in a gravity fed brush with a small nozzle, to 40 psi for thicker paint and a bigger nozzle. You need to experiment and see what works for you. I generally use 15 to 20 psi with my brushes. I do believe that you should have a regulator though, otherwise you'll have no control.
A question that I have seen several times on forums is whether you should set pressure with or without the trigger pressed. Pressing the trigger on your airbrush will probably cause the regulator reading to drop a pound or two. It really doesn't matter. The main thing is to be consistent so that when you find a good pressure, you can return to it later.
What about a moisture separator? Yes, I think that is a good idea, especially if you live in a humid climate. Without one, you could get occasional drops of water in your paint job, which is usually not a good thing.
Postscript
I picked up a box of food colors at the market; four colors, red, yellow, blue and green. They can be thinned with water and combined to make any color you want, even brown. They really are great for practice. I've been coloring the comic strips from the daily paper. It's a great way to develop trigger control. Clean up is just a flush with plain water.