Hi Susan, I agree the best is to use sewing thread. But, on my RAG quilt where the stitching shows so much, I used contrasting embroidery thread on the top and bottom, then when I clipped it, I clipped near to the thread. I like the sheen of the thread. I don't think regular bobbin thread is strong enough... Poly is fine, but I don't know about rayon. The last thing we want is the for thread to break, or shrink more than the fabric. Thank you so very much for making a quilt for the orphans in Uganda! Can't wait to see it! hugs, Jan