Cannon opening "weathering"

I would use black chalk. The sell them in art supply stores and Michaels. Make sure the pastel is actually chalk and not of the oil kind. Some people grind the chalk with 600-grid but I find that scraping it lightly with #11 Exacto blade is just as good if not better. I use makeup aplicators to apply it. It is crucial that you test first on a scrap piese of plastic or paper towel to get a feel of how you want the streak to appear. And then it is the dreadful moment. Still, I prefer this method to airbrushing largely because I don't have good control of my airbush for spot/streak simulation.
Hope that helps. Nice picture.

Borislav

you can also drybrush it with a medium fine brush.

Drybrush? Ok, here comes my newbiness: I thought dry brushing was used to highlight high spots and detail?

As for using chalk, I keep forgetting about that option. Thanks guys.

Dana

When the cannon is fired on aircraft, the weapons troops will clean the barrels right after flight. It is just like the gun cleaning solvent you would use to clean your firearm. The Crew Chief or Plane Captain would then use a solvent to clean around the gun blast area. They would try to remove as much residue as possible. The reason, it was found late in WW ll, that on unpainted metal the gun residue was very corrosive. If allowed to remain on the aircraft it would eventually eat into the aluminum and cause severe pitting.

This practice carried over when the aircraft started receiving camo paint. That is still part of the check list to clean the area after firing of the gun.

If you wan't to show an aircraft just after flight with a dirty gun fire residue, then dry brushing is the best method. Heavy at the opening and lighter as you go back. Don't forget the gun purge exhaust as well.

you should try pastels or a special powder made for paintings of saints in churches.