You are absolutely right to be skeptical. There are a great many downsides to this technology. Getting it grimy, pointing it out of the sky, cloud cover, etc. will nullify any cooling effect. And yes, the DIY preparation shown would be completely destroyed by a powerwasher, or even a moderate storm. That said, I read recently of a university of maryland research team made an improved coating, allowing the nanospheres to withstand significantly increased weathering while still remaining effective.