i have used those over-head-plastics. it is not bad if they are written all
over. i just let pupils paint them black (with ink?) on the other side and
white (with latex or gesso?)on the other. then the gesso side is nice to
scratch on. you could also vary the color combinations on the sides of the
plastic(yellow and different violets or several blues and some oranges?).
the lines in scratches are personal and depending on the thickness of the
raili from finland.