I pretty much wash all my cottons before sewing. Simply to remove factory oils and to shrink the fibers. If you will dry clean the final garment then you don’t necessarily need to pre-wash the Rain No Stain. (Can’t remember if I did or not but I know I used it on an elaborate bustle skirt and did not pre-wash for that. If it’s for petticoats – definitely pre-wash.) And don’t be afraid to wash twill. It will shrink and doing that before you start working with a hot steam iron on it will save you grief later from patches that shrink and others that don’t due to where & how much steam ironing you apply.