Simon, that delightful hut reminds me that I have still got to tackle the SR Exmouth Junction concrete footbridge kit (superbly laser cut in 1/32nd for us by Woodbury Models - nudge, nudge, plug, plug!) for Bagshot!
The problems with that are twofold though:
Firstly, the bridge has several "faces", so will need to be painted before final assembly - with some handling risks as well as potential adhesion issues?
Second, How on earth do I best represent the aggregate "show through" that you have just mentioned?
I partly comfort myself with the knowledge that there would have been significantly less of the "shingle" visible when the structure was still relatively new than we generally like to think! Evidence of that fact was clearly visible on the weather protected undersides of the prototype.
Just look at how smooth - and creamy coloured the raw finish is! It is worth noting that this particular example has been standing since the Ascot to Frimley Junction line was electrified in 1938!
The hut appears in the posting as it is, and to be pretty damn close to this finish already?!
Finally, this view shows the contrast - and probably more like that which most of us would expect it to look like?! Yet this effect is entirely contemporary -
and the result of almost eighty years worth of exposure to the elements!!!
My real fear is that it would be too easy to inaccurately overdo the paint job - but then again, if I were to do the bridge as it should be for the late fifties or early sixties it might look slightly too clean and oddly under cooked?!
Anyway, I have had a few thoughts on the technique:
Way back, I had to frequently use an airbrush for commercial illustration work. The original, infernal instrument is hiding somewhere amongst my junk - complete with it's special "spatter cap". I used to set the air pressure low, fit the cap and get a lovely even speckle effect on parts of the artwork where required!
When it comes to it, and if I can't find the aforementioned, I may resort to the old trick of simply puffing with a bit of tube across a loaded paintbrush - although that is much, much more difficult to control the spread!
I wonder if lightly blowing a gloss varnish instead of any specific colour, using either method might be ideal for this and your hut job?
Might be worth a trial...?
I will be very interested Simon, as it looks like you will be having a go before I get round to mine!!
Pete.