Wash board behavior observed

I observed bees that have taken up residence in an upside down flower pot doing this strange behavior. I put the word out to my bee friends and the consensus is they are applying wax to the bottom of the pot to make it waterproof. Bees have been observed doing this on certain parts of their boxes. How do they know to do this? They are such interesting creatures!

https://www.youtube.com/watch?v=EavTZt8ybDc