Hey there,
I think you should use delrin finishing bushings. There is one member called johnnycnc who sells them, with those you won't have to separate the tubes of your pen with the bushings, they won't stick together as they are smaller. Usually when I am done, I just sand the ends of my pen tubes to make them straight as glue can build up. That should take care of that white line problem. ;-)
C
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