Isn't that the whole point, though? To put a promotion in a place where it won't be washed away? Some of these events are being promoted a month in advance. It would suck to have to come and redo it every 2 days. Using chalk on a covered surface is the perfect combination of permanent and temporary. And regarding the art, I'd be fine with it staying there for a while because it looks awesome. Especially the robot.
Also, blue star to the WCL and Wehner idea.
Also, blue star to the WCL and Wehner idea.