I think I mean any period when stock was subject to routine use and maintenance. The purpose of paint was to protect the materials from which the wagon was built from decay - rot in the case of wood, rust in the case of iron and especially steel. And anything anywhere near a moving part would have been coated in grease - buffer shanks, axleguards where they guide the box, springs, hinges, etc.
Also, looking back at those Didcot photos, I'd be very wary of taking any of the unpainted woodwork in their buildings or permanent way as a guide. Timber used in such places would have been treated with creosote, which isn't allowed nowadays.
Another thing that I've seen overdone is brake block dust. The hand-operated brakes on ordinary wagons could not be applied with anything like the force of a vacuum or air brake.
I realise I've been dragging a very old set of posts out into the open, scraping off the rust and greasing them up! Not sure how I chanced on it...