I use the Gutterman polyester thread in the large spools used for sergers. I buy it at JoAnn's with a coupon. It is nice and fine so quilt seams lay flat yet it is strong and doesn't break easily. I am currently using it to fmq on a quilt that I don't want the stitching to show up a lot. I saw a presentation by the President of Superior Threads and he convinced me the old thought that Poly thread would cut the cotton fabric is not true.