Apologies for the tardy reply - a bit of ill health has intervened but all sorted now.
Exteriors - Back in 2008 I was finishing up the camo on the (then) current 1/48 incarnation of the Airfix Mk.I that drew much flak from that dedicated bunch of Airfix bashers, after tidying the bits that were the basis of all the furore. Pre-shading of panel lines was all the rage, along with panel line washes and this was often utilised to such a degree that end results were starkly 'geometric'. It clashed with what I was observing in reference images. It was clear that when you looked (when you really looked), weathering followed certain rules (airflow, gravity and so forth) but was also comprised of multitudes of tones, shades, marks, scrapes, scuffs and other surface imperfections that didn't follow panel lines or structural shapes but were random, asymmetric and organic.
I was ok with an airbrush but no expert, so I sat at the bench pondering the principles of 'random, asymmetric and organic' and how I could go anyway towards breaking up my Spitfire's factory finish. Then it hit me - I would 'scribble'. Like trying to get a biro working again on a bit of paper, except the airbrush would be loaded with a thin, lightened version of the base paint. This is what I wrote in a forum article -
"Now the weird part - I disengaged my conscious brain (my wife questions whether it ever gets into gear at all...so do I some times) and without thinking rapidly traced the thin spray over the Dark Earth only in a random, aimless zig-zag, up, down, side to side, all over the place non-pattern - pulling away from the surface, dodging closer - you get the idea. The key is don't think - just 'do'. Switch off the targeting computer and â€˜â€¦use The Force Lukeâ€¦.' Anyway, I'd like to go back to my cell now?"
This is what I wrote in Airfix Model World about 'scribbling' in 2014, as I'd applied it to a test shot of the 1/48 Airfix Spitfire Mk.Vb -
"â€˜Scribbling' with the airbrush was a technique I first devised at the bench back in late 2007, early 2008, as a means of disrupting base colours before the application of other methods layered over the top.
The approach was simple - load the airbrush with thin colour (no more than 10-15% paint) and run a fine spray continuously and randomly over the surface in sections (i.e. one wing upper surface, then the other, then a portion of the fuselage and so one). The height of the brush was varied during each â€˜burst' (lasting five to ten seconds each), from very close to an inch or two. Pauses were made to check progress before continuing on and no mind was paid if it seemed over heavy here and there as this was easily rectified later with the same concentration of raw base colour.
It was essential to blank the mind and avoid conscious control and to think of the airbrush as a biro that had stopped working, which was being randomly scribbled on paper to re-start the ink flow. The paint mixes were lightened more and more heavily and the normal number of stages was three or four before swopping to a deliberately darker shade like Tamiya X-19 Smoke, to add darker marks.
The essence therefore was not to â€˜draw lines' per se but to allow suitably thin paint to intersect where the scribbles criss-crossed each other and create marks and little swirls. Thin paint, a little confidence and some practice would see your finish disrupt into a satisfying base for further weathering."
That summarises the principle and it's what I routinely lay down as a disrupted base for oil colour staining and here, not all oil colours are created equal. Winsor & Newton are the common currency here but I've been converted to Michael Harding oils - hand ground pigments are significantly finer and yield washes that are 'creamy' and not granular in comparison.
The oil staining process is simple, just damp a section of the model with white spirit (mineral spirit in the US), take a chisel ended brush and damp it with a little wash colour (a lighter version of the base colour - or darker, complimentary or contrasting - you decide). Dab the colour on the model here and there. Let it bleed out and dry (or dry it with a hairdryer from a distance). Keep the surface level as you apply the dabs initially and tilt a little to alter the stain distribution. Seal the first layer of staining with a light coat of Klear (Future in the US). Apply several more layers of staining, each sealed as before from that to come, building one upon the other.
I often 'dry sponge' Humbrol enamels over base paint (on lowers as well as top sides) to give thousands of contrasting marks. These can be left hard edged or softened and smeared with a chisel brush damp with clean white spirit dabbed or dragged over the marks. A light spray of base paint over the top will further blend in marks and of course, you can oil stain over the top.
So, the broad m/o is base paint / scribble using several or multiple shades / dry sponge shades as desired, softening if desired as you go and perhaps lightly over-spraying / oil staining in layers, sealing between each.
Metalwork - Please see my explanation on page 2 for Alclad II plus oil staining to create a metal patina.
Cockpits - Not sure why I included this as my m/o is simply to work as neatly as possible but chipping fluid on painted seats is a fun way to depict heavy wear but beyond that all the exterior principles can be utilised inside with dry sponging and oil staining.