I just finished a redwork top that measured about 48 x 48 and I, like kathyd, used muslin to back the blocks. I basted around the redwork and the edges to keep it smooth. A local quilt shop has a material they sell for backing redwork although I don't recall what the name is but it feels like flannel. The lady in the shop said I could use flannel instead. However, since muslin was less expensive I chose that. I'm not an expert in redwork, but it seems to me that any fusible no matter what weight would make it stiff and make the needle sticky. The whole idea is to use something to hide traveling threads and knots, which according to the redwork police you aren't suppose to make knots, but I haven't figured out how to keep the thread from coming loose if I don't make a knot -- so I make knots. If it were me, I would find some inexpensive muslin the same color as your background fabric instead of any fusible product.