Recently I inherited a dozen or so mineral wagons made up from 1960's vintage Airfix kits. Whilst they were very well made, the builder never painted them but did add the transfers. In an attempt to bring them up to modern expectations but without the additional work of stripping all the transfers off I decided to weather them as I do my buildings. When performing this, I drybrush on various greys, rust reds and then dust on grey primer and matt black from Halfords aerosols. An effective and quick fix - just another 10 or so to do!