I'm a teacher and write on a whiteboard throughout the day, wiping it off as I go. There's no money for anything (I bought the pens) and we had one bottle of liquid cleaner in the cupboard. I used a small amount and wiped the board down and it took off all pen and made it look shiny and new.
Then I wrote all over it again, tried to use the dry erase rubber and it was like the ink had stuck. No matter how much effort, it smeared, you could still see the words and couldn't write over it clearly.
I tried again and the same happened again. I can't be using up the whole bottle every time the board needs wiping down! Is it designed for this to happen, so you have to buy more? Does it just not work? How do I lift the residue off??