I have these tubes of paint for whiteboards... Like 6 different colors. I think I got from Really Good Stuff or someplace a few years back. I want to use it on my chalkboard... and I'm wondering if anyone has done that... I am testing it today in a little area... and it does wash off.. but I'm worried that if I leave it on (I want to use it to create a calendar on the blackboard) it will seep in or something... and leave a stain... Any thoughts? Thanks!