Several photos I have here show with 100% certainty that the interiors of some open wagons in revenue-earning service of some pre-Group companies (specifically the GN and GE) were left unpainted. However, I have a note (tertiary source, so treat with due caution) that the LSW painted the interior of opens with 'one coat of lead paint' on release to traffic. My understanding gained from snippets of information so far generally suggests that many revenue earning general merchandise and mineral opens had unpainted interiors. For example, Cassell's Cyclopaedia of Mechanics (1900) describes in detail the process of painting a railway wagon, but only mentions the exterior. One reason suggested as to why this might be is that a painted interior would promote rather than hinder the planks rotting. On the outside of a wagon rain water would run down off the planks and fall away. However, the badly damaged and abraded paint on the interior of a wagon in service would allow rainwater to creep and settle behind it keeping the wood damp rather than allowing it to dry out. A second reason is the financial implication of almost doubling the not insignificant cost of painting a wagon, signwriting costs excepted. The cost of paint rocketed in both the mid-1880s (so much so that the GER, now long out of chancery and doing rather well thank you very much, ceased painting the whole of the ends of goods brakes vermilion, limiting the colour to the headstocks only) and then again during the Great War evidenced by repaired GE wagons being released back into traffic with new planking left unpainted on the exteriors. To balance the above and going against the grain (sorry!), Sidney Stone's 'Railway Carriages and Wagons' (1903) describes how wagons are 'plain painted' - the paint mixed was with boiled oil (which weathers better than raw oil) and he states that: "The principal work which may be comprised in plain painting are underframes, outside of roofs, floors, ironwork attachments and wagons throughout."
Stone was Asst. Loco Works Mgr. of the GCR with quite a pedigree at the time his articles were published.
Edit: shabby writing.