The attached .skp file contains one group composed of elementary items (lines, surfaces). The group item is in layer “old door”. All its lines and surfaces are in Layer 0. I cannot remove layer “door”: it supposedly contains something. Eneroth Layer Usage Inspector says layer “door” is used by the group item. How?
All surfaces in the group item are painted the default color. Yet I cannot remove two in-model textures via Purge Unused. Why?
I must be missing something, but I have no idea what.
The group “door pegboard” is hidden using its hidden flag rather than a layer. You can right-click it in the outliner and select “Unhide”. Then I think your issues will go away or at least be clearer.
Unfortunately, I guess, I’m still obtuse. I deleted that group “door pegboard”. That allowed one in-model texture to disappear. But both problems still exist, with the remaining single group. So I’m still looking for a reason why.
Remember that even edges can have some material assigned to them. This may inadvertently happen when you explode groups or components. An you will not even know by looking at all the faces. Purging won’t get rid of these materials.
There are also some edges and faces inside the group that are associated with layer0. They are the edges that form the inner surface of the lock mortise. You can make them visible by view->hidden geometry. You can fix their layer assignments by opening the group for edit, selecting all via triple-click (model info will show a blank in the layer panel because the selection uses a mix of layers) and then choose layer0 in the model info layer selector.
And, as @g.h.hubers noted, these edges also have a material applied to them.
I applied the other texture to the group as a whole, which must have applied it to the back faces. I then applied the default texture to the group, and also to all items in the group. Why didn’t that remove the other texture from the back faces? Aha! I had to apply the default texture separately to the insides and the outsides, even though Ctrl+A selected both the insides and the outsides. Interesting.
As far as the hidden geometry: I wonder how that happened… (Don’t answer: rhetorical question.) Overall moral: unhide everything before jumping to conclusions.
Unfortunately that’s not the explanation. Materials applied to group containers do not get applied to the faces inside, not even the back faces. Generally you have to work a little harder to apply textures to back faces. Probably the most common way for the back faces to get painted with a material is if the faces were inside out at the time the material is applied.
Generally it’s a good idea to get the modeling nearly completed before you start adding materials. Materials can hide issues with the geometry. Make sure face orientation is correct as you go and make sure all faces and edges remain on Layer 0 (Keep Layer 0 as the active layer) and when you do apply the materials, it’s usually best to apply them to the faces and not the group or component container. This requires opening the group or component for editing before painting.
Adding to what @DaveR wrote: many of us set an outrageous color for back faces in the style and use monochrome while building a model. That way any reversed faces jump out at you before you encounter these kinds of issues.
