Even though the car is pretty grotty, I thought that the driver might make an effort to keep the windows clean. Clean white spirit has been used to remove the Dullcote layer. Soaking a cocktail stick (wooden variety) in the white spirit will allow a gentle rubbing action to gradually remove the dried Dullcote a little at a time, and if you don't go right into the corners it can leave an accumulation of 'dirt' there. The tiny flakes that will be left adhering to the plastic windows through static electricity can be removed by wiping with a cotton swab or a blast of air from the airbrush.
This would have been the last stage for this model, but recent information received suggests that there are some areas where rust should be, and isn't. More to come. . . . . . . . .