Of course, Clint! If I may, I'll cut an paste the process that I documented on a previous build...
With the post-shading done, I will go to the next weathering step: pastel wash. This will be done using two colors given the color differences between the lower and upper surfaces. The bottom surfaces will be given a dark brown wash to highlight the recessed panel lines and rivets. Here is the bottom prior to the wash.
My thinned paint finishes are on the delicate side so I prefer using a wash that is benign to the paint finish. So I make my own water-based wash. It's very easy... all you need are some pastel chalks (not oil), water and some dishwashing soap.
I put a very small amount of water into a small container. For the bottom surfaces, I select a dark brown and a black piece of chalk. Using a knife blade, I scrape fine amounts of both chalks into the water.
Here are the ingredients prior to mixing. To help reduce the surface tension of the water and allow easier mixing, I add a small dap of dishwashing soap. Note the small amount of water that I am using.
Mix it well and you should get a nice, solidly-colored fluid. This small amount is enough to cover the entire model and then some.
Using a small paint brush, the wash is applied over the bottom surfaces, making sure that it gets into every recessed line, raised detail and rivet. Let this dry COMPLETELY.
I like to give my washes about 10-15 minutes to dry completely so that it holds in the places that I want it to hold. Wiping off a still wet wash will result in too much removal and you'll have to repeat the application. Once the wash application is dry, I begin to wipe it off using a piece of damp (not wet) paper towel. The dark pastel pigments will remain in any recess, serving to highlight that feature. Always end your wipes in the direction of the airflow.
The wash is systematically removed. Depending on the effect that you want to achieve, you can leave residual streaks of the dirty wash on the model to simulate wear.
Remember that crazy mottling pattern that I applied to the model before the camo painting? That was the first layer of effects that have been applied to the model. The pre-shading, post-shading and now the pastel wash, all work together to create an illusion of depth on the otherwise flat surface.
