Thanks guys, I appreciate the comments.
Yep, Guppy that oughta do it fine. Should be plenty of references there.
thebest1, not sure how others do it but I us a variety of irregular 'spash' type brushe in PS, and use an opacity of less then 50% and just build it up slowly with 2 or 3 layers and silver/grey shaded paint, then when it looks right I compress them into one layer.
I never think it looks totally right; I usually get as close as I can to what I'm thinking and force myself to stop. Otherwise I'd still be tweaking it now.