"What we're doing is something that's not necessarily illegal, but also not necessarily legal, it's a little bit in a gray zone," said GreenGraffiti founder Jim Bowes.
From a distance, an ad on a San Francisco sidewalk might look like paint or stickers, but a closer look reveals it is actually made of nothing.
"We put down a stencil in a dirty spot, we pressure wash the lettering and lift it up, and there's the clean advertising," said Jason Roberts, a power washing contractor.
It's advertising created by cleaning -- otherwise known as "reverse graffiti."
"You know, if you write 'wash me' on the back of a car in the dirt, that is actually reverse graffiti. It might have been the first form of reverse graffiti," said Bowes.
Bowes started GreenGraffiti and gave us some video shot as his company "cleaned" ads for big brands like Range Rover, Sony and Starbucks all over Europe. He sees it as a greener and subtler alternative to billboards.
"We're not using any paper, we're not using any ink, our messages aren't being backlit," said Bowes.
In fact, from start to finish, the whole thing is done with water.
"The pressure is so high that the stream is moving supersonic, it's about mach 2," said Kevin Binkert, a Standard Metal Products owner.
And that little ferocious water jet is strong enough to cut through steel, carving out a stencil for the clean ad. In about 20 minutes, the logo for clothing company Patagonia is ready to hit the streets.
"You know, I'm not using any harsh chemicals, just the water and the heat and the pressure," said Roberts. "The dirtier the better."
That's enough to blast away months' worth of sidewalk grime to create the letters on a background of dirt.
Reverse graffiti as an art form is not new to San Francisco. Back in 2008, a graffiti artist from London came and "cleaned" a giant mural into the gritty filth on the wall of the Broadway tunnel.
"They go up and they look at it and they think it's spray paint or something, and then it's just a cold realization that the world is really, really dirty," said Paul Curtis.
Curtis goes by the name "Moose." He was captured in a short film, crafting the 140-foot mural that was commissioned to help promote a cleaning product -- Green Works.
"It's what we call a larger than life product demo where you can really see how well Green Works cleans," said Ria Lacher, a Green Works brand manager.
Though sidewalk ads get covered back over in a few weeks from being walked on. People don't walk on walls.
"We're almost approaching four years. It's still here," said Lacher.
Well... almost. The Green Works logo has been wiped away. Though Moose had a permit to create a mural, he didn't have a permit for outdoor advertising.
When asked if there is any circumstance in which the city has allowed people to put ads on a public right of way, San Francisco's Department of Public Works spokeswoman Gloria Chain said no.
Public Works is charged with keeping the city's sidewalks, streets, and even the Broadway tunnel, free of what it calls graffiti.
"Graffiti is unsanctioned, unpermitted work that's been put on either buildings or the public right of way," said Chan.
And that's where it gets tricky.
"We're not applying something to the surface, which is how most of the rules are written," said Bowes.
Bowes says so far, he's never gotten in trouble because most governments, including San Francisco's, have no laws against cleaning.
"We're going to be investigating and taking a look at what enforcement code this falls under," said Chan.
But instead of citations, Bowes wants to see cities selling permits for clean ads.
"I believe that people would tolerate advertisement on the ground, in this form, if they knew that a portion of the money coming out of that was actually going towards funding schools or funding the national forests," said Bowes.
And while the city figures out how to handle these new ads, it's asked the companies responsible to come back and remove them, by cleaning the rest of the sidewalk.
