Heres one of a scrap line class 47 I did for a member of this forum ~

It uses several techniques, including the use of salt to create the paint blisters on the cab roof, dry brushing, grime washes and paint fading to create a very patchy finish based on a class 47 that was dumped in a local scrap line for almost a decade.
A more basic weathering job on a kit built van~

The van was built from a Parkside kit and initially painted with a base coat that was a mix of grey and brown (I forget which was used) to represent unpainted wood. A few bodyside planks were selected and scratched to produce a wood grain and then masked before the normal BR Bauxite livery was applied over the top. Then it was just given a light dirt wash and the body framework and chassis dry brushed to pick out some of the detail.