Thank you very much!

@unknownuser said: There's a plugin that converts components into groups somewhere in the Plugin Store The Inverse Groups in Components yes by Thomthom but this one ? You have Loose to groups by Chris Fullmer but this one ?

Since that link doesn't directly point to your answer and someone else searching here may want to know, the materials don't appear because you painted them onto the back faces in your second example. The OBJ export only shows materials on the front faces.

Sorry, I am new to meshlab, also. I just downloaded the other day and all I have been able to do is read some of the information from the site and from Wikipedia. I know from other reading that somewhere along the way during transfers, this polygon problem will pile up on you. I think you were talking about coplanar faces. I took this to mean you have a bunch of superfluous edges subdividing faces. There are some render apps that do not tolerate triangles and others that do not tolerate quads. Frankly, I am bumfuzzled. But, since this topic is started, you will almost certainly get more qualified advice.