i use hb pencils. and sometimes the lines are REALLY dark, gone over a few times.
i have never had a problem washing it out, even on white on whites. i finish up, say a prayer and go to the laudromat. tide in the huge washer and i have been golden every time.
i have a drawer full of marking gadgets i never use. i am also very very suspicious of any ink that "disappears" because chemicals and fabric? never a good combo.
aileen