Giving your models a realistic 'weathered' appearance is easy. Start by rubbing an artists' pastel pencil (appropriately coloured) onto a piece of sandpaper, then brush the dry powder onto the model with a stiff brush.

For best results, this technique should be applied last, after any coat of Humbrol Modelcote varnish you have applied, to cover the paint scheme and decals in the areas you wish to 'weather'.