Thanks for your help Jeffrey.
I’m gonna give you an example with colour, it’s the same with material of any kind.
By default, everything in your model is neutral. depending on your style, neutral is white/grayish, or something else. My default theme uses a bland light/dark blue.
As soon as you apply a material on a face, you tell sketchup “this face is not neutral anymore, now it’s… let’s say green.”
When you apply a material (green, let’s keep green) on a group or a component (I’m gonna say group later, but it’s on both really), what you say to SU is “ok, now, inside this group, neutral = green”. In your general file neutral = neutral, but not inside the group where it’s green.
Therefore, all the neutral faces of your group become green. Now if you enter the group and select a (green) face, you’ll see it’s actually neutral (check entity info). and since neutral = green, it will look green. If, while inside the group, you paint a specific face, let’s say red, it stops being a neutral face, it’s now red. Therefore, the neutral=green thing doesn’t apply to it. it’s not neutral it’s red.
Off course, this idea stacks if you have a group containing a group containing…
When should you paint inside or outside ? as Dave said a few years back, by painting outside the group, you abandon the material edition tool. so off course, if you’re using colours or materials like gravel and grass, it’s fine. it allows you to paint the whole group quickly and then enter the group to paint details with something else. Or paint everything face by face.
with materials like tiles and brick, being able to align material to faces is important, so you’ll probably apply it face by face.
90% of the material trouble you’ll get with the 3d warehouse come from that (source : trust me). You’ll grab a couch, component, containing several groups containing several groups containing faces. You want to remove the ugly material of the pillows ? find where it was applied (at what level).
or delete the material and repaint the now neutral faces (I often do that, download the object, strip it bare, and repaint it myself).
or use an extension like fredo thrupaint
or stop using the 3d warehouse and model everything rom scratch like a 3d warrior