I use them and the white line disappears on the dark fabric after washing, at least mine did. Don't starch or use ironing spray on the marked lines. The pen is non toxic so nothing in the ink will harm fabric. I put a piece of white muslin marked with all colors of the pens then put in freezer after I ironed the marks off and washed it with detergent. The marks did not come back. Even if they did come back in the cold, I would just toss in the dryer and not worry about it.