I've been using 0.7 mm or 0.9 mm mechanical pencils to mark on plastic blinds.
I guess my plastic ones are always shaded by plants becoming overgrown, and few or none are more than one year old. Maybe that's why they don't crack.
Maybe the older ones have gotten somewhat brittle ... I'll be on the lookout for that when I bring them back inside to erase them or wash them.
