When I try to erase my dry erase board there are always marks left on it. Students cand see the work that we did in previous classes, and I'm always scrambling to try to get the board clean. No matter how hard I scrub, there are marks. I am doing a lot of problem solving, and I don't want classes coming in to see how to solve the problem before they try it themselves. I have expo cleaner ($$ too expensive to use as much as I need to), but I'm wondering is there anything I can do to the board so that it erases cleanly.