I've been sewing hexies for quite a while now, but instead of using a whip stitch through multiple layers, I use a ladder stitch to sew them together. I use more than 18" of thread at a time, too, and I haven't had any problems (well I don't as long as I remember to dangle the needle to unwind the thread periodically). The stitches don't show, which I like. I do use Thread Heaven.