I have used flannel as backing on several quilts. Washing in hot water is a must, perhaps even a double washing is in order. Also, wash dark colors, with a color catcher. My DD had a piece of red flannel she wanted to use on the back of a baby quilt. She decided not to use it because after washing at least 4 times with a color catcher it was still bleeding. I also look for good quality, portugese, little used flannel sheets at my local thrift stores. They make great one piece backings.