Typically I weather my models using artist pastel chalk. With the chalk you can vary the depth of your weathering. For jet exhaust I used black, brown, light gray, and a touch of blue. Using multiple colors gives a nice blended look. The “tools” needed are cotton swabs, eye make-up pads, brushes, and foam applicators.
First you need to decide on what color to use for highlighting. Light colored paint it is best to use a shade or two darker. For dark colored surfaces use lighter colored chalk. You want to highlight or dirty the surface to give it a worn/used look. Cockpits and interiors have lots details that can be brought out. Here is an A-7 cockpit that I used a darker gray chalk.
Here is the main interior of a FW-200 where I lightly dusted the areas but used an even darker color to highlight the details and the walkways. I used a lighter color to highlight the battery.
Exhaust staining is another typical area on aircraft. I like to use a brown base then black, and finish off with a lighter gray. This gives the appearance of staining as well as the highlights of the heat of the exhaust.
For jet engine exhaust I add a bit of blue. The metal used on exhaust will blue a little from the extreme heat. In the photo below The top shows the exhaust just painted with Model Master Jet Exhaust paint. The bottom show after weathering. You can see how the chalk builds up and adds a level of grime to highlight the detail.
Here is a recently weathered F-16C exhaust using black, brown, and light gray chalk.
Finally I use it to highlight the panel sections. You can vary the areas of wear by either applying more chalk or using a darker color. One area to make darker is the walkways on the aircraft. Areas around the engine and fuel ports can be highlighted then using a damp cotton swab it can be streaked to look like liquid staining. In engine cowls, using a dark brown and a damp swab you can simulate minor oil leaks. Once all the weathering is done you will need to spray a light coat of overcoat to seal the weathering. The end result will add a touch of realism to your model.
Weathering with pastel chalk can be used on any type of model. Here is a photo of a 1/350 aircraft carrier flight deck. I went very heavy with the weathering on the high traffic areas.
You can find sets of pastel chalk and various hobby and craft stores. Look for sets with a large variety of colors. The set that I use is available from Amazon (see below) and has many colors.
Amazon Link: Pastel Chalk