The hotel proved a bit of a challenge. The original model was only divided into the glass (doors and windows) and the rest. After much experimentation I found the best way to deal with it was to open in Blender, break into separate objects (much easier to select faces in Blender than in Mari), export as obj groups, which then gave the selection groups in Mari. I had originally tried just painting the different colours, not in a selection, and it proved a nightmare. I think there was a mismatch between the quads and the pixel grid of the paint buffer. Blender has much better selection tools for geo. And it’s a lot more responsive than Mari too, which I find quite sluggish.