(Somewhat) Weathered Windows

LEFT: Window primed with Tamiya Tan and Hunterline Driftwood wash. RIGHT: Window primed with Hunterline Light Gray. CENTER: After drybrushing white. 

I've long been frustrated at the results I've gotten painting and weathering plastic window castings. I'd find more often than not the walls on my models - even the styrene ones - would look pretty good. However, the plastic window and door castings always looked like fresh and newly painted. That might be acceptable for a well-maintained building, but what if you want things like windows and doors to show the ravages of time? 
The logical approach would be to paint the plastic castings white and then weather them. After all, that's what happens on the prototype. Honestly, for darker windows it's not as much of a problem. But most windows, no matter what the color of the building walls, tend to be white. So the trick becomes weathering a light colored window. 
I tried airbrushing those white windows with some earth tone paint. And I tried washes of India Ink and alcohol, pastel chalks and drybrushing colors such as Raw Umber and Grimy Black. The results always ended up looking like someone had streaked, airbrushed or chalked a dark color onto a white window. And though I took some solace in the fact that my results were consistent, they still looked awful. 

An Experiment
As I was painting the windows for the implement dealer it occurred to me that the solution may be to reverse the "paint final window color then weather" process. 
After removing most of the sprues and gates from the windows, I taped them to a scrap of cardboard on a roll of painter's tape (the Green Frog kind - but the blue tape works just as well!).
Then, instead of reaching for the can of white spray paint, this time I sprayed everything with Tamiya Flat Tan. After that dried, I gave the castings a quick wash of Hunterline stains. I found Light Gray (shown here) and Driftwood seem to produce a pleasing "soft" warm almost-but-not-quite black color. 
After I waiting for the stain to dry completely - more than 48 hours in the case of the doors and windows in these photos - I went over each with a light drybrushed coat of white paint. A little goes a long way here - and I found a light touch is required. I did get acceptable results after a few practice attempts (see lead photo above and the door photo below). 

If you can't tell, I'm quite pleased with the results.