WEATHERING CARS - STEP 1 FILTERS

I use my own cobbled together method for weathering freight cars.  Generally, it’s three steps - 4-5 coats of various filters,  oil washes and highlighting, and finally dry powders.    For filters I use Vallejo Air colors, thinned about 3 or 4 to 1 and applied with an air brush.   Generally I do 3-4 coats on body and then turn upside down and do two or so coats from under, spraying upward to simulate dust and dirt from the roadbed.    My go to is a medium grey for the car bodies, and a dark earth for under frames.   The photo shows identical cars, one with three coats of a grey filter, the other nothing.  It illustrates how well the filter fades the bright colors.  

Since I model the PC era, I take into consideration the dates on the cars, and the use of the car to determine how much or how little weathering.