Don't Rely On Decals
Allright, I can hear you asking: Why on Earth paint any markings when you have decals for just about anything? True, the choice of quality decals is very good these days. Still it may be not enough if your intention is to replicate a particular aircraft not directly provided by the decal/kit - manufacturer, or if you encounter any problems with the decals at hand.
While providing a great way to include color detail on the models, decals also have a number of disadvantages. I can think of several reasons for a decal to be less than ideal solution.
The first potential problem is availability. Do you have a decal exactly matching the desired marking on your model? Are size and proportions of the markings right? How about the shape of lettering? While custom-made decals can be made using the graphics software and a special printer, there's a great deal of markings that can also be painted directly on the model. And even custom-made decals may posess other decal deficiencies described below.
Decals inherit the limitation of the printing process. Some colors are extra difficult to replicate correctly in print; tricky shades of camouflage colors usually fall victim to these limitations. Taking the World War II RAF colors for example, the usual source of headache is the Sky color of squadron letters. I also find that the reds and the blues can be reproduced in print with only a limited depth of color, causing many national markings to look quite lifeless. Swedish AF fans will know how vivid is the correct blue color of the Swedish roundel - and how rare are the decals that resemble the correct shade.
A major problem that I'm sure most readers will be familiar with is that all decals are more or less transluscent. For decals applied on dark backgrounds, the dark shade will invariably show through, spoiling the intended impression of color.
Also, decals do not work well on complex surfaces like spinners, noses, edges, as well as surfaces with sizeable moulded-in detail.
Decals need to be of good quality, and they require good preparation. The reaction to decal setting solutions, clear decal film showing after the application and sometimes silvering are the three variables which I find difficult to control unless having previously worked with decals from the same source. Given that decal application takes place towards the end of the modelling project, it is all too easy to spoil the overall result.
Lat but not least, bad quality decals my be printed off-register, may curl or break beyond repair during application and so on.
Summarizing, decals are great to use if of excellent quality and available. If not -- well, either your demands are modest and the available decals can be accepted, or the rest of this article might be of interest to you.
Enter the high school of masking: to paint markings you will need masking material able to produce pefectly straight, uniform and knife-sharp color edges.
Your best friend will be a good quality masking tape. I can recommend using Tamiya's own masking tape, which I have used myself and found excellent. However, I have also been using stock paper masking tape from a hardware store with prefectly acceptable results. The quality requirements for the tape are fairly simple. The tape must be able to adhere very tightly to the masked surface, but at the same time not peel the underlying paint - or indeed decals - on removal. It needs to be thin and elastic to follow the curves of your model. It needs to be pre-cut on a piece of glass without loosing its adhesive abilities. If, like Tamiya, it also has the prefectly straight edges, it will save you a lot of extra cutting.
I can also think of using other fine masking materials like soft vinyl foil or Bare Metal Foil, but I have not tried these.
As for the tools, you will need a piece of glass or a ceramic tile for cutting the designs, a sharp hobby knife, a steel ruler and a high quality measuring compass. A Q-tip might be handy to press down the edges of your masks.
Before commencing any real project, you might also want to practice your painting/airbrushing technique to ensure prefectly sharp color edges with no unneccessary build-up of paint or - God forbid - the paint runing and creeping underneath your masking. I find that using some extra thinner and airbrushing in multiple coats of thin, quickly-drying mist works best for me. Usually I also carefully remove all masks while the paint is still wet to prevent any paint build-up along the edges.
Since much of the masking and painting must be done in stages, you will win much time by sticking to acrylic paint for your markings. When thinly applied, acrylics will dry to the touch in a minute or two, allowing you to mask and spray several consecutive steps of the design in a single airbrushing session. With oil-based enamels, you will usually have to wait overnight between the steps.