When I use Steam A Seam II on the back of my applique the directions say to wash fabric to get sizing out. I put my fabric in a bowl of water with a drop or two of dawn, swish around a little with my hand, rinse and roll up in a towel to remove excess water, then hang on hangers using clothes pins till dry. Then iron, then fuse. I tried to fuse without washing but it wouldn't stick properly. Finally decided the fusible manufactures new what they were talking about. It takes more time to start with but pays off in the end. I don't pre-wash any of my other material.