This is what I'm going with. I've read about five other articles and they all say about the same thing: https://www.wikihow.com/Restore-a-Whiteboard
Sigh. Chalkboards (real slate ones) never wore out. And the chalk never went dry half way through use. And quality chalk didn't squeak or crumble like the cheap stuff.
I taught two terms of intro biology to ESL students in college. I bought my own chalk and had a chalk holder (because I hated the feel of the stuff on my skin). Worked like a charm.