alternate opinion alert. I *know* I *should* prewash flannel. I don't. I don't prewash anything. I just washed a quilt that was flannel on the back and it came out fine. No bleeding, no excessive shrinkage. And you can mix fabrics, most just don't. And mixing on the top is as okay as using a different fabric/fiber on teh back. Do what you like. That's the point of this hobby/avocation/passion. at least, IMHO