Already sewn together???? Not sure - but i've always added white vineagar to the fabric prewash - and whether it works or i'm just lucky - i've never had a quilt "run" when i used the fabrics. I just don't know if that would work in a finished quilt. There is a product i have heard mentioned here --- the name starts with an S.....but i have not used it. Someone who answers will know. GOOD LUCK !!!