Thanks so much for the comments! Happy New Year everyone... can't believe it is already 2018!
The gloss coat has sealed the camo, markings and decals so we are ready for weathering. The first step is to give the model a wash. A misnomer if there ever was one since the objective is to dirty up the plane, not clean it! The wash also serves another important purpose in my eyes. I want remove the 3D aspects of the panel lines and rivets. In other words, I want to convert the physical grooves and holes on the model into lines and dots or at least fool the eye into thinking it is seeing lines and dots, which is what the panel wash will do. I believe this "flattening" effects helps to unify the model's surface into a more substantial object.
I make my own liquid washes from pastel chalk, water and a drop of dishwashing soap. Being water-based, this wash is very safe on most finishes and can be wiped off completely if necessary, even from a flat coat.
Here is the bottom of the wing, before the wash is applied.
The wash is applied by brush into all of the rivet lines, panel lines and any other raised or recessed detail.
Let this dry completely. If you get impatient and start wiping before the wash completely dries, you'll be removing the wash from the areas where you want it to stay. You can tell the wash is dry as it develops a totally flat finish. Using a lightly damped piece of paper towel, I wipe the wash off the model, leaving the dried liquid in all of the nooks and crannies. Initially, I try to keep my strokes perpendicular to any panel lines but the finishing strokes should be in the same direction as the air flow.
I like to work in areas as it is easy to lose track of the wash applications. The last thing you want is to spray your final clear coat onto an area that you forgot to wipe.
I use a lighter color wash to treat the black tulip area on the nose.
Make sure you inspect the model carefully during this process for stray fingerprints, unrealistic streaks and any areas that may have been missed. If you are using a paper towel like me, make sure you wipe or brush the model off since paper towels can leave fibers behind.
The wash effect will vary on the contrast between the wash color and the surrounding color. On the lighter RLM 76 surfaces, I used a dark brown wash but I'll use a bit darker wash with more black on the darker RLM 75/83.