Is a wash a substitute for pre-shading (do they accomplish the same purpose)?:
A pin wash is a substitute for pre-shading at least for me.
No, not really. The idea behind a a pre shade is that it makes the underlying paint look dirty. Or it makes the areas not pre shaded look more faded. The pre shade changes the tonality of the base color above it. Pre shading makes the paint look faded, worn out or dirty.
The pin wash brings out the panel line details by making them stand out a little bit. It doesn’t really change anything about the base colors that its being applied over. You could pin wash a perfectly clean aircraft (no weathering) and it would still look very clean.
The difference between a dark wash and pre-shading, for panel lines, is the effect. The pin wash only darkens the actual panel line itself. Pre-shading darkens the edge of the panels also. I personally only like this for removable panels- I don’t see that effect on full sized planes all that much for other, non-removable panels.
I do use an overall wash to impart a griminess to the build
I’m not saying that using a wash to simulate griminess isn’t possible. I just did it with my C-17. But a wash and a pre shade really don’t accomplish the same thing…especially a pin wash.
Depends on what the modeler wants to accomplish. I used to pre-shade panel lines to darken them, but nowadays I just use pin washes.