My sewing machine dealer told me I should use only poly, top and bottom, but they were not specific about the thread weight. They suggested Mettler, but that is all they sold. I've read about many different threads while "lurking" on this thread in order to learn all I can about quilting from those with long term experience. I've done a few quilts (complete with hand quilting), and I never had a problem with melting while ironing.