It depends on what you’ve used for the color coat. I paint with enamels and so I usually do a wash with 50:50 acrylic/water mix with a drop of dish soap in it. You can also use oil/turpentine mixture but I tend to use acrylics. Brown shades are better than black shades.
I’ve done it using pro-modeller wash. Apply the wash, let it dry, wipe off using a pulling motion in the direction you want the streak to go.
-Fred
You could also try using pastel chalks to achieve them. I have done simular effects by mixing whatever colours you want and then using a small brush streak it in the direction of the airflow. Start small and build it up to the way you like it. If it all goes wrong then wipe it off with a damp cloth and start again.
Steve
[#ditto] except that I’d use Conte Crayon powder, applied after the final matt varnish coat. Sticks better than pastels, and just as easy to remove with a putty rubber if you don’t like what you’ve done, or thnk you’ve done too much. The weathering on the underside of this AM 1/48 Il-2:
was done using Conte Crayon dust, as was this Revell Germany 1/72 Hurricane:
If you want a hard edge on, say, the forward edge of a panel line, simply mask it off with a post-it note. I imagine you could achieve soemthing similar with weathering powders, but they might be more difficult to remove.
Cheers,
Chris.
Thanks all for replies; I just wanted to make sure this can be done with pastels rather than using some fancy airbrushing tricks. I have lots of pastels in all kinds of colors and an application stick. I will try this on the inside after the final matt varnish.