Before making a start on any discolouration, the model is given a good seeing-to with Testor's Dullcote. This immediately removes the high gloss finish that is found on diecast models. The windows will be rectified much later on - at the end.
The rigger brush is dipped in clean white spirit and then into the wash (it's an enamel wash) , and the tip used to apply the thinned wash to various areas of the model - wheel centres, engine bay, rear bucket and a couple of areas where detail shows through the heavy layer of paint. The photograph was taken while the wash was still wet, but it dries to a satin or matt finish, depending on the level of thinning achieved to start with.
You could leave it like this, to portray a vehicle not used very much, or kept reasonably clean.