Based on some of the engines I've worked on over the years the progression went from painted wood, to stretched canvas that was painted and sealed over wood, to painted sheet tin over wood, to finally steel sheet with a wood interior under-layer. What time period we wish to replicate would govern the type of roofing finish we apply to the engine. Since we are replicating early 20th Century railroading I would recommend wood (sealed) with an adhesive top coat followed by stretched heavy canvas and topped with a really good iron oxide based paint. I would suggest a "marine" grade to better handle the elements over time.
