How interesting that you brought this up. I just took my sewing machine in for a service, and the lady at the shop checked the spool of thread I had on it (just all-purpose thread). Then she very seriously warned me never to use dual duty thread because it has a wax on it that builds up in the workings of the machine and affects how it works. I have never heard that before, and have never had a problem with dual duty thread. Has anyone else heard about this wax coating? Or is it another "myth"?