Some quilters confirm that Flannel bleeds when washed. Flannel combined with other fabrics may bleed towards some parts of your quilt which do not bleed. It is possible that a flannel backing can bleed toward the top part of your quilt and would make it look not very good. So, before any of this destroys the quilt you have poured many hours on sewing, you can put in a procedure or two before you start cutting and sewing. Pre-washing your Flannel to sew prevents it from bleeding after you have finished your quilt. You should note that when your quilt is finished, it will undergo a lot more washing. If you are not a pre-wash quilter, then maybe you should start thinking otherwise, especially when using Flannel for your quilts. Or to be sure, you can test your flannel fabric by soaking a small piece in warm, soapy water and wait for 30 minutes. If the water is still clear after the allotted time, you will have no bleeding problem. If the water becomes colored, you can pre-wash the fabric then, use a color fixative like Retayne, which helps retain the color on the fabric.