I have always prewashed my fabrics, for garments and quilts. And since I've been piecing quilts, I prewash the fabrics I'm going to use for a quilt altogether with the Retayne and hot water. I include "Launder in cool water only" on my labels. I don't want someone to have the red dye into the white and ending up with pink and etc. after all the work is done. My sister-in-law made a blue jeans denim quilt and added wine colored blocks with it. After the first wash, it was a miserable looking mess; even for a jeans quilt. Lesson learned for her. She thought she was 'saving time' by not prewashing the wine colored fabric. I haven't ever used the sythrapol.