Forgive my ignorance on the subject but what does washing your model mean in the context you guys are talking about?
I've been toying with the idea of revisiting my childhood hobby of model making as a teaching tool for my kids so I'm genuinely curious.
A wash doesn't mean "soap & water" (though washing your models before you prime them is often recommended, that isn't what we're talking about).
A wash in this context means: after the paint job is final, putting a finishing coat on it to add shadows and such.
A wash can be a watered-down color or it can be a special finishing coat. In any case, the idea is that a darker coat (and this can be brown, blue, grey, black or something like it) seeps into the cracks around the model and darkens them. It creates a higher contrast and brings out certain details. The wash also will cling to the edges or raised parts of the model such as a hatch, to help define those better, too.
This article seems to explain the concept and also provide some decent before-and-after images:
http://www.scalemodelguide.com/guide/painting/paint-washes/
