I use acrylic washes as my "primary technique" for weathering.
And I mix the washes for each weathering sessions - and vary
the mix of black, white, red oxide, brown oxide each time.
In addition, I vary the amount of wash applied to the individual
cars in any one weathering session. And some times I am
doing only 1 car and other times as many as 5 or more.
Finally - I often vary the mix of the colors in the wash(es)
during the session ... usually, but not always, working from
the darker shades to lighter ones later in the session.
I also almost always do some "dry brush weathering" that
involves applying weathering colors to the "details" such as
the fans/grills on a diesel, the trucks, the parts of the model
that are metal on the prototype.
And, of course, the under body, ends, and roofs are treated
differently than the sides, etc., etc., etc.
My objectives/goals for weathering are pretty simple to
1) Every piece I do "isn't finished" until it gets some level
of weathering. That varies from "just" dull coat to a
level that is referred to as "heavy weathering". (Yes,
that "every" includes locos and passenger cars and
cabeese and plastic and brass!)
2) When you look at "a yard/train comprised of all of my
cars" if you
a) take a quick glance you would say "they all look
the same" and "they are all weathered" (especially
if they are STMFCs)
b) take the time to look/study more you would say
"yes, there are fairly large differences in the
amount of weathering from car to car" ... and
you would be able to identify the individual cars
as having been in some particular service or
part of the country ... where appropriate.
c) in depth study of the cars would reveal different
techniques and/or fairly large differences in the
way a particular technique has been applied.
Those are my goals. I'm sure many of you share them.
I'm also certain that many of you have different goals.
And we are "all correct"!
P.S. Go back and look at that pic of the train in Colorado
in 1958 ... the first glance says "the cars in this train
are all weathered" ... then with just a bit more study
you see that some are actually fairly new and some
make it even hard to tell what road they are ... and all
the cars near the power (around the curve) look
"all the same" and other than knowing which are
box cars and which aren't there is very little other