A 40 wt thread is pretty thick for piecing. Also as Patrice mentioned, stay away from Rayon for piecing. It is very weak thread and meant for decorative purposes such as embroidery, it isn't really intended for utilitarian use where any real pressure or torque is put on the thread (such as piecing seams). My piecing combo is bottom line in the bobbin and Aurifil 50 wt 2 ply cotton in the top. If you prefer all poly there is no reason you can't use bottom line in both top and bobbin. My friend who is a precision piecer and show quilter uses Wonderfil decobob in both top and bottom which is a very fine 80 wt polyester. I piece with only two colors of thread. A light taupe for lights and a dark charcoal for darks. A short stitch length and fine thread keeps any thread from showing in the piecing seams.