I recently used Joann's premium muslin and ran into something that had not happened to me before. I used the premium muslin before just fine. I did not prewash it; I don't prewash anything. I also use 505 spay baste. For some reason the spray would not adhere well to the muslin. Maybe that particular batch had chemicals/sizing in it that made it different. I started quilting and that backing just shifted around. I ended up ripping out the quilting and re-sprayed and tried again. The same thing happened!! After ripping it out the second time, I used a different fabric for the back. I washed the piece of muslin and have used it for scrap now. So long story short...I would prewash that backing. I am now a little leery about using their premium muslin at all now...sad :-(