As I do not currently possess an airbrush, I decided to purchase some Humbrol Acrylic Spray and make an attempt at attacking some wagons.
I firstly mounted the 'Mainline' Cambrian wagon on a doner Bachmann chassis, then renumbered it. It has been weathered soley with Humbrol Dark Brown No 29 with matt varnish added.
The Dapol 6 wheel milk tanker has been done in the same way, with a bit of Carr's weathering powder added. As a first attempt I don't think they look too bad.