Hello everyone! 
I've created a model of a large crate in SketchUp and exported it as an OBJ file. I've then imported the OBJ file into Maya 2017 because SketchUp has this annoying habit of not having the vertices connected for some starnge reason. So in Maya I've joined up the vertices using the Merge tool and exported it as an OBJ file again, replacing the one I imported.
I've then used UV Layout Pro to do the UV Map and saved the changes to the OBJ file.
I need to create an ID Map and convert it to an FBX file because it seems that the developers of UV Layout Pro are the ONLY people on the planet that work with OBJ files, the world and his wife just use FBX files. So I've imported it into Maya and exported that file as an FBX file.
Now that I'm working with the FBX file I want to assign materials of different colours to different parts of the model to create my ID Map. So I've done this, screenshot here:
But as you can see the UV Map is gray which is the original material that the model had before I started adding new materials. Now, if I go to face mode by right clicking and selecting Face and I click on one of the coloured faces the entire UV Map changes from gray to whatever colour the material I've clicked is. Here in the screenshot below you can see that I've clicked on a face with the red material and the entire UV has gone red.
Have I done something wrong? Shouldn't the UV Map reflect the colours on the model? When I've brought either the OBJ or FBX file into Substance Painter and pressed the Bake Maps button you can just see several different colours filling a square rather than the ID Map that I want.
I'm completely new to all this sort of stuff so any help will be much appreciated, many thanks in advance for any help given!
