One of the most amazing things about building armors is to give them a worn appearance.
Somebody of us prefer to leave our models as they come straight from factory, others choose to decorate them achieving an effect as if they were in combat.

The first step is to apply the base color correctly, the more thinner this layer is, it will show to our eyes less volume. This is important because on later processes of washes and lightened we will add layers of paint that will increase the overall paint thickness on the model.
For this reason when we apply the base color with airbrush, we should diluted it as far as possible, 50% or more.
It doesn't matter if the first and second hand are almost transparent, so much better, we will be able in this way to be sure that the finishing will be the thinnest possible.
If the model is camouflaged, the process is the same one, you must apply the smudges in thin layers, very well diluted.

The painting

On the showed model, a German army's Panzer III model J used during the Russian campaign in winter, we talk about a simple decoration.
After the building has been finished, we begin the painting process applying a well diluted light gray layer, to highlight the imperfections that could have the plastic surface.
Then we apply the tank base color, in this case we use the dark gray ( Humbrol 27), although this color is slightly more light than the panzer gray panzer, but this valid, because we want to represent a tank which has been much time in battle, the gray lost intensity and it looked lighter.
After using the Humbrol 27 gray, we will reproduce the winter camouflage.
Although I say camouflage, it was only a white paint layer all over the tank, it was a way to make their detection difficult in snowy landscapes. Then we have to use dull white, Humbrol 34.

Although this stage seems simple, in fact is the first step to achieve the weathering. Since the white painting provided to the troops was washable, (most of the times) we can observe on pictures of that time that the tanks had the white painting peeled off, showing the gray base. To simulate this effect we close to the minimum the airbrush and softly we will paint on certain places selectively, not to cover the gray base entirely.

Now we begin the weathering with brushes. After having painted the white color, we will simulate detachments in this color.
Either the camouflage or the worn out details have to be done following original photos and the own modeler's taste, to give it more realism .

One of the most interesting and difficult things about the aging on a model is to give it relief and depths, on the body, hinges, tools anchorages, motor cover, and so forth.

Shading and lighting

For the shading I used "Koh-inor" chalks, I sanded them to obtain a powder that I deposited over the tank with a brush, as if it were a make-up.
Other modelers prefer to make this wash with acrylic paint or oils.
Once all the surface has been decorated as you wanted, the powder should be fixed, I used alcohol, but acrylic varnish can be also used.
For the lighting I applied the dry brush technique with a snub and square brush in select areas.
Lastly, using Humbrol 27 ( base color) I did small scratches on the white paint to imitate the photos I was looking at.

Conclusion

In this kind of decoration, the most important thing is to let our imagination fly, trying to visualize in mind what we want to achieve, it is also important to collect pictures of the time and / or of the models built. This is a technique used in plastic arts.