Those who had chance to see military aircraft, know that there is only one time when they look "brand new", when they come out from the assembly line. The passage of time and the use, deteriorate the external painting giving the airplane an aging aspect, remember that many of them fly at supersonic speed and/or are exposed to extreme environmental conditions. If you are building your first model, you will surely be doing it out the box, but you can achieve incredible results, and give them a characteristic aspect of "wear", with very few materials and a couple of simple techniques.

The process the seasoned modeler uses to give this look to the model is called weathering. Many times it is represented on the model by the soot of the exhaust, oil stains, chipped painting, etc., all that can really give the model an amazing authenticity.

When we apply weathering techniques to a model we have to use the same logic used by mother nature. For instance, we shouldn't put smoke stains (from the exhaust) and then apply a gloss decal over the blackened surface. Usually, airplanes are well preserved, they don't show dents, but smoke stains start appearing around the exhaust exits, as well as oil drops, on some surfaces. Many times chipped paint can be seen on the wings leading edges and stabilizers, and on those places where the crew or mechanics walk. Anyway, we have to keep in mind that all airplanes with a good maintenance will show minimum amount of wear.

Two basic ways to show exhaust stains
The first way is with the use of an airbrush, this requires practice to achieve the desired effect, but it's always advisable because the results are excellent. We begin mixing three parts flat dark gray an one part flat brown (3:1 ratio). We will apply this color with the airbrush adjusting needle very closed, spraying from a distance of 2 cm approximately, moving from the exhaust exits to the back, trying to follow the exhaust gases path. It is convenient to practice this on an old model or on a piece of gloss paper, before working on our model. Then we will apply flat black in the same way over the first color.

The second method is using watercolors, crayons or chalks on the black to gray range. We begin grinding this material into a fine powder, then we will apply it on the model rubbing it with an old paintbrush. On the exhaust outlet the color should be dark and thicker. The method for application should be the same that the one used with the airbrush. The result of this technique is not permanent, so it will be necessary to apply one or two coats of dull varnish or lacquer, to prevent the stains from disappearing.

Oil stains
This kind of stains are usually very light so we will have to use very little color. (Oil really has very little color so it only leaves light stains)
We should color thinner with some drops of black paint or ink in a 10/1 ratio, we allow a small drop to fall on the airplane surface area on which we want the oil stains to appear, then we will blow the "oil" drop along that area having in mind the direction the airplane flies, the movement should be from front to back. It's not convenient to exceed with this kind of weathering, so it will be enough to apply this technique on one or two places.

Chipped paint
Simulating paint chips is a simple technique, but like others is easily overdone. A military airplane wouldn't have many chips; they usually appear on the cutting edges of propeller blades, on the leading edges of wings, on flying surfaces, and on the areas where the crew or mechanics walk or  they  work. (i.e. on the wing roots). We should work using aluminum color to paint the chips, applying them with a fine pointed brush with little paint, we apply the chips in small dots, the smaller the better so, a very large chip would look unreal. Airplanes with control surfaces covered with fabric (i.e.: ailerons), must have a different treatment because the chips are not too visible, due to the flexible and porous nature of the surfaces, making it difficult for the paint to scratch easily.

Another very common technique, but for advanced modelers, consists on the double layer paint. We apply a thin layer of silver or aluminum paint, it should cover the whole model, then we use masking liquid (applied like white glue as liquid dries off, and it is easy to be removed) we cover small areas in those we want the chips appear (as we made with the silver paint in the previous technique). When all is dried we usually apply the color the model requires and after several days, we remove the masked chips using masking tape or the point of a well sharpened blade, the effect showed is very real because the silver area appears under the color, pretending to be the exposed metal.