I had a red fabric that I prewashed in Retayne. However, when I completed my king size red and white quilt top I discovered that the red bled onto the white when I spritzed the marking for quilting. Synthrapol removed the bleeding stain after 3 washings. After having it about 3/4 finished with hand quilting another accident happened -- the cat. The red bled again. I decided that it was time to cut my losses on this "Murphy's Law Quilt", and I tossed it in the trash. There are some fabrics that just won't stop bleeding. Now, I test my fabric by cutting a small square (3-5 inches) from the corners of the darkest and lightest fabric that will be going into my quilt. I place a few drops of the soap I'll use to wash the quilt (I wash all my quilts after finished) in a bowl or glass of water. I drop both pieces of fabric in and let it sit all day or overnight. I check to see if there's color in the bowl and if my light fabric has absorbed it, I try another light fabric. If there's no color on the light fabric, I rinse both pieces, lay them together on a flat surface, lay something flat and heavy on top (I use a heavy cutting board), and let them stay together 24 hours. If the light fabric hasn't absorbed any color, I use both fabrics. If it has, I discard the dark fabric for that project. I learned this from Harriet Hargrave's "From Fiber to Fabric." It is time consuming to test, but better and less frustrating than my first experience.