Speaking from experience, as I have finished 7 old tops, I have found muslin on the back works great, as that is something they would have had at the time. I also use cotton batting, hobbs or quilter's dream. I will use a waxed off white quilters thread - or I have used a green shiny poly embroidery thread for my most recent finish on a grandmother's flower garden for some extra sass. I alway wash when I finished and dry it for the old fashioned shrink. The results will be worth the effort. I promise.