Hi Rob,
I know you are asking for larger scale (rather then 54mm) but Missing Lynx did have a tutorial on painting it using an piece of shredded sponge.
I feel that this technique may not translate up to your scale.
I've seen recommendations for the salt techniques (usually used for chipped armour) or airbrushing through a mesh, probably photo etch for this scale.
Truth be told, if you come up with a method (or two) you may be the first one to share a tutorial.
I suspect that the only way of getting a good result for a 120 or larger is going to be to do the hard slog - painting it meticulously by hand with a brush, making careful reference to photos as you work.