I'm not a long armer, but I do use metallics sometimes. I haven't used this one, but if I were you I would check it out:
http://lindeegembroidery.com/a-new-m...-arm-quilters/
I like Superior threads, and their sites says they work with long arms:
http://www.superiorthreads.com/produ...FSNp7AodbhcAFg