Have you thought about breaking down your coach bodies into parts that best fit different materials my Siphon J (https://www.shapeways.com/shops/stafford_road_model_works?section=GWR+Dia+O31%2F40+Siphon+J&s=0) comprises Black Natural Versatile Plastic underframe, White Natural Versatile roof (the roughness can be smoothed by applying several coats of Halfords primer and sanding back) and Smooth Fine Detail Plastic for the body sides/ends where fine detail and a smooth finish is more important?
I hope to produce a "proper" coach sometime this year using fine detail for the sides only, probably breaking the body into three parts (solebars+ends, sides and roof) fitting them together along similar lines to the old Hornby-Triang Mark 1 coaches