Washes are exactly what the name implies, a wash of grimy paint, usually thinned down with in our case, 50% water, and then “washed” over the aircraft. This really works beautifully in replicating how the elements streak a slight grime over an aircraft and adds to the believability of the model. It also provides plenty of opportunity for “happy accidents”- things you didn't plan for, but end up looking great.
I started with the bottom of the aircraft. Airplanes, even civilian ones, get dirty from exhaust and mud from the runway. Since this a model of an aircraft based in Florida, I rationalized the runways would be wet quite a bit and there would be a lot of muddy water splashed on the underside of the wings and fuselage.
My technique involves first wetting down the area to be “washed” with a spray bottle. I also rub the water in to the surface to break the water tension. This allows the paint to “bleed” and creep freely along the surface.
In areas I don’t want the paint to go, I dried with a paper towel. I want the dirty paint to collect after the raised area behind the gear door. The paint will bleed into the wet area and not the dry area...
Like so.... This looks like way too much paint but not to worry, when dry, it will be much lighter. And also, if you don’t like the way it is going, you can always wipe it up before it dries.
Paint color: For a muddy mixture I used Model Masters Acrylic Burnt Sienna, a little black, and a little white to “gray” the color down a bit. Again this is thinned with 50% water.
I tilted the tail of the model towards the ground so the paint would flow in a way to mimic the natural airflow of the aircraft. Keeping in mind that the prop blows air in a CORKSCREW pattern around the aircraft, I let the paint drift sideways to replicate this effect.
Airflow goes to the left somewhat due to prop-wash, but splatter from the wheels would splash outward.
With acrylic, you can speed up the drying process with a heat gun or hair dryer. You can also guide the paint with the dryer as well!.
Here I am applying a muddy wash to the flaps that is noticeably more pronounced than on the wing. Why? Because the flaps are down a lot of the time when the plane is on the landing roll out and would capture a more direct flow of dirt from the wheels and prop.
Here are some detail shots of the end results. Should look great as the model flies overhead.
WASH ON TOPSIDE OF AIRCRAFT
For some reason, the accepted practice is that fuselage washes run VERTICALLY as opposed to the wings which wash HORIZONTALLY with the airflow. I chose to stick with that convention. It seems that aircraft would probably be on the ground more than in the air and if left out in the elements, would have vertical streaking of grime from rain and the like that is more pronounced on the fuse.
First I misted water on the area that will be accepting the wash, rubbed it into the finish to break the surface tension, then applied the wash and let gravity do its thing. To speed up drying and to “lock-in” the paint, a heat gun works wonders.
On the fuselage, note that the grimy wash is NOT flowing with the vertical panel lines. Since we are simulating the weather effects of the plane on the ground, I set the fuselage on the appropriate angle as if it were sitting on its landing gear. Note how soft the edges of the grimy wash are due to wetting the surface first. This also prevents the wash from running right down the surface in rivulets.
To help things along and direct the wash the way I want it, I used a damp paper towel and pulled the watered where I wanted it. Recessed areas kept the wash and raised areas were wiped clean. Just as in reality! This technique does all the work for you.
Here are some shots of the results...
Notice how subtle this effect is but yet it is powerful. If it were darker, it wouldn’t look natural. I think this looks about right.
The rivets really pay off with this technique, since they retain some streaking of the wash directly under them leading to a very realistic effect. (The slot in front of “FIRST” is for a foot step on the aircraft. I painted some steel enamel here to simulate the paint wearing off from the side of shoes rubbing in this place.)
When washing this area of the wing, I dripped some of the grime wash heavily around the gas cap to simulate leaks and venting of the gas down the wing. Note the “rubbed off” paint near and around the wing walk. I also used a little Flat Tan acrylic to simulate dirt left from the shoes of mechanics and pilots in the appropriate areas.
One of the neat things I like about this process is that some “happy accidents” happen now and again. On this model some of the grimy wash ran under the aircraft and made an interesting run on the bottom scoop. Interesting that it mimicked the very thing I saw on a reference photo of the scoop on the actual aircraft! Cool!