When I prewash fabric I use my regular detergent and a regular cycle on the washer and dryer. I don't know why you would need to purchase something different. I've been laundering my fabrics and finished quilts for 40+ years, sometimes I might use a short wash cycle but I've always used my regular detergent. If it's ( smelly) fabric I sometimes add vinegar to the wash cycle to help get rid of the odors.