What’s a “wash” and why do I need one?
A wash is where a mixture of paint and thinner are combined. The mixture is mostly thinner with a few drops of color added (Testors gloss black in this instance); how much color is added is determined by how strong you want the wash to be (that will make more sense shortly).
The reason for applying a wash is due to what our eyes (and, more importantly, brain) does with light. We see because of reflected light. What we see has “shape” because of shadows and highlights. Where the light is reflected directly back at the eye is a “highlight,” and the areas that don’t reflect light are shadows (of almost infinite intensity). The engine being replicated here is large. But the replication itself is small. Both “large” items and “small” items do different things with light even if their shapes are identical. So the way to fool the eye is to create shadows where there really are no shadows.
That’s what a wash does. By thinning the hell out of it, it doesn’t cover as much as it collects. “Cover” should be obvious but what the hell does “collect” mean? When the wash is applied, because the color is so thinned out, the wash collects at places where surfaces/planes meet, and a properly thinned wash doesn’t much care how small these surfaces/planes are. When the wash collects in the crevasses and cracks, what it does for your eye is to convince it that what you’re looking at is actually larger because only large objects treat light like that (well, small ones do, too, but the effect is not very noticeable).
And of course there’s a trick to all of this. If the color you’re applying a wash over is acrylic (as is the case here), then the wash has to be enamel. If the color having the wash applied is enamel, the wash has to be acrylic. The reason for this is because the substrates are different, they don’t bond to each other and that property is what allows the color to collect instead of coat.
Once the wash has been applied and allowed to dry, then the piece being treated is given a coat of clear (matte, semi-gloss, or gloss, depending on the effect you want) and it’s all sealed.
How that plays out is like this… I started with a matte finish (top photos) and ended with the washed finish. Notice how the photos on top look like something small and the photos on the bottom don’t? That’s what a wash is for:
The next step is to apply wear (for this I used Humbrol’s steel again), dry-brush the wearing and chipping, and then overshoot the whole thing with semi-gloss. And hopefully now when you look at it, absenting anything else you know the size of for scale, you can’t tell what size it is…and that’s the whole point.