2.8 : Trouble with materials when joining two meshes Blender novice here, using 2.8; I've got this .blend file holding a multitude of shapes; the resulting object will be imported in Unreal Engine.

Topics are convering this already, but I'm so unfamiliar with Blender that "add an input > UV map node" isn't helping much; I'd be very gratefull to have a more detailed, step-by-step approach. Keywords, bullet points and I can google the rest. Learning with Udemy at my own pace doesn't help you to identify the solution "the clean way" myself.

Attached is a cleaned-up version of the file, with only two meshes and their own material plus the texture they share:
joining-example.zip


First step was to join the two meshes. Ok.
Next, I must find how to "make them render properly".


There's where my language gets crude. I understand it has to do with UV maps, but can't find what's wrong. See below.

Before joining
Each mesh is properly rendered.



After joining
Only one mesh will be properly rendered (the last selected)
By default, the texture node uses the first UV map in the list. When you join the two objects the two materials find two different UV maps in the new object, but both are using the first one, which is good for one material but bad for the other. So simply tell the second material to use the correct UV map, using an input UV node.

Don't forget to apply (Ctrl A) rotation and scale to both meshes in object mode, before joining.