The thing that strikes me is how muted the colours appear. Much "weathering" is done by adding new layers of powders or whatever, but really the underlying colour has faded in the elements.
If I was having a bash, Richard, I’d mist on a coating of flat white acrylic or enamel with an airbrush to start, or a translucent oil paint (China White?) if using a brush which I’d soften with further soft brushing if I didn’t have access to an airbrush, to create the faded paint look. This would be followed by a ‘wetted’ application of thinned oils or enamels - perhaps even some of the proprietary products say by AK Interactive - to create the streaks, applied with a long bristled brush with most of the hairs removed.
I’m not sure I’d bother with powders for this, unless it was part of the ‘wetted’ application mentioned.
jonte