I used polyester (not cotton wrapped poly) to piece a top. When I finished putting the last rows together and pressed everything flat it all looked really great. As I was running my hands over the top to smooth it on the sandwich my hand went through the seams. On closer inspection, most of the seams were no longer stitched as the poly thread had melted under the iron. And the iron never touched the thread, just the fabric. Thus I do not use or recommend poly or rayon thread for piecing.