basically what a wash is what you want to do to give a custom a certain weathered look, I consider it the "bizzarro-dry brushing". What you do is dilute your paint (I don't know the ratio though, so someone else will have to take care of that) with water, and dip your brush in, then stroke the brush all around the custom -- when it dries, the water wil have evaporated and the paint will have not settled directly where you painted on, rather it will settle and dry in all of the nooks and crannies of the figure/ship/whatever, and give it that dirtied up look. I've seen some really nice customs that were washes and it really does bring out tyhe detail on the sculpts.