Jon,
I have been looking for a reference to the painting of early open wagons in a 'lead' colour internally, which I recall from somewhere. It may have been an LSWR specification, but if I find it shall pass it on.
For what it's worth, the attached photos show a S7 wagon I built a few years ago, using a pretty straightforward technique for the wood colouring. Planking was scored first in the normal way, and whilst the components were still separate, each inner surface was rubbed over with coarse sandpaper to form some surface texture. The occasional piece was also worried with a scriber , trying not to overdo it.
A single coat of cream paint was then applied and allowed to harden off. When fully dry, thin shades of brown were rubbed along the graining with finger in a rag, all to taste, but the work of a moment.
Internal detail pre- painted black was then affixed after judicious scraping back of the paint to give the solvent some purchase. Similar scratching away at the joints allowed the main pieces to be assembled as normal, then the interior masked was for spraying on completion.
I haven't tried it on a G3 model yet, but it might work, especially with the internal strapping added with functional bolts etc.
. I built this wall some 25 years ago and always thought that it had a gentle curve to it but that was before I tried laying track et al. This whole experience has shown me that I didn't build a curved wall at all but a wall with curves joined by straight sections 
. If I had known all those years ago that I would need it for a garden railway, I obviously would have tried much harder. 
