My "goal" for weathering is
"when you look at the yard everything looks the same - but when you look at
individual cars and compare them to the ones next to them they look different".
In order to accomplish this I simply never do any form of "batch weathering"
and work on each car as though I had never seen it or any other car before.
Yes, that also includes going back and re-checking photos of real trains/yards
and photos of other guys work ... such as this excellent B&O box car.