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Our new office has the white board paint on one wall of every conference room. Facilities had to add stickers to those walls so idiots would stop marking on the other ones. Idiots because it is easy to tell the difference. The whiteboard paint is slick and shiny where the other walls were a flat paint.
We have something similar and we had a slightly different problem. We'd be drawing all sorts of elaborate and fancy sh*t on the boards and it would all be gone the next day. At first we thought there was an idiot prankster but it turns out the cleaners had been told to wipe them down by their manager. So now we have to have signs that say "DO NOT ERASE".
Keep your friends close. Keep Kill your enemies closer. The End
My rule of thumb: at the end of the day, take a picture.
Since conference rooms are used by many groups, do not assume that what I put on there will be there when I get back, and, unless the room is specifically and exclusively assigned to my group, putting a "Do Not Erase" or "Please Leave On" is just more for someone else to wipe off.
After all, if someone else is using the room, they may need access to the white board as well.
When I started secondary school, there was a blackboard with a chalk map of Vietnam and surrounding countries on it with 'Do not erase' written on it. I do not know how long it had been there, but it was still there when I left several years later because no one was brave enough to erase it in case it was still needed. I wonder if (many years later) it is still there.
We have some portable white boards that came from the old office, they still have drawings on them from well before they were moved. One drawing in particular was the, at the time, new desk systems that were going to be installed. Three stations in a pod all facing each other. The desks didn't follow us in the move. Wish they had.