There are few things more frustrating than finding that you’ve used your chalk markers on the wrong surface. If you’re not sure that a surface is non-porous you can save yourself a lot of trouble by testing it. First, find a place on the surface where you would not be upset to see a mark. Then use a liquid chalk marker to draw a small line or dot. Use the darkest color out of the ones you had planned for your project, one that would be likely to leave a shadow if you were unsuccessful in completely erasing it from the surface. Then be patient. Wait at least 24 hours to give the color time to be absorbed if the surface is porous. Liquid chalk marker on some porous surfaces might wipe off completely after a few minutes or even an hour but may leave residue after several hours. One example of a surface you may want to test first is a chalkboard since not all of them are non-porous.