Aircraft is very clead, shiny and tidy machine. Yes, but only in moment when it's leave paint and finishing facility in the aircraft factory. After testing plane is delivering to the operational unit. Under permanent stress panels are not as tight as they was, color bleeding under sun light, mechanic walk on plane, surface is detoriating, fitings and connections begin to leak, oil, fuel and various of other liquid's... in short word it's areal life. So let's bring life on illustration.
This is most interesting part of artwork. Simply you select part to part of the plane and paint something what present gas exhaust, damaged color, dirt and so on. I gues what- you need new layer for this. Used color are grey brown with various shades, ranging from pale to dark. But take attntion about the brush- it is airbrush but in Dissolve mode and opacity range from 40 % up to 70 %. Place color stroke arond the edges. let there be dark color. Then use Smoodge tool, R from keyboard and find under pallete, and distrort the stroke to create leaks in the direction of flight. Then take again brush, take very light color and apply on the edge of the panel. Then take Blur tool and take blur on the whole section.
And this have to be done for the each part of the plane. Somewhere you will put gun blast traces, but somewhere just damaged paint. see the how the paint could be damaged on the cockpit canopy.