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I want to delete all materials on all meshes and only add one default material to all meshes? I tried the "Copy Material to Others", but it's not working.
EDIT: OK, I got it now. The problem was, that some objects were hidden. So the "Copy Material to Others" didn't worked. When you open the file, then first press "Alt + H" to unhide all objects. Then do the "Copy Material to Others". Save file and open it again.