If you are piecing I would use a 50 weight thread. I use Master Piece by Superior Threads. Aurofil also makes a nice 50 weight thread. The suggestion for a smaller needle to avoid leaving holes is a valid one. Try some samples after you have gone to a finer thread. If you are getting loops on the bottom you must either tighten your upper tension or loosen your bobbin tension. I would suggest tightening the top tension first. Even though your Saphire automatically adjusts for tension it is possible to override those settings. Check your manual or with your dealer on how to do this. Automatic cannot adjust for every circumstance. Be sure to write down how you changed it so you can put it back to normal when you are done with the batik.