I use to never prewash UNTIL a lady gave me a quilt top she has been working on and got tired of. It was a complicated design done in black and white. I finished it and then washed it using one Shout color catcher and cold water. She had told me she had prewashed the black. When I opened the washer, I was shocked. I had a dark grey and light grey quilt and it was not pretty. It was ruined. It looks like it is about l00 years old but in my eyes it is ruined. Now I test all material and then wash it before starting to sew.