The potent one-two punch is how the Maryland Transportation Authority keeps tiled walls in the Fort McHenry and Baltimore Harbor tunnels glistening from the beginning of April through Thanksgiving.
For the overnight scrubbing operation, workers mix the anti-soot soap with hundreds of gallons of water and place the solution on the backs of two bug-eyed, German-engineered trucks called Unimogs. Each of the four McHenry and two Harbor tubes — nearly nine miles in all — goes through the wash-and-rinse cycle about once every six weeks.
It's not vanity. Clean tunnel walls reflect vehicles' taillights, giving drivers a heads up on stopped traffic. That's considered especially important in the McHenry tunnel, which has a big bend in the middle to accommodate the Fort McHenry National Monument and Historic Site and a dip to provide clearance for the shipping channel.
Nor are the Unimogs, with their huge blue brushes, considered an extravagance.
"We used to wash the Harbor Tunnel with soap and water and a rinse truck. But there was no agitation — no brush — to scrub the dirt away," said Chris Allison, tunnel maintenance supervisor. "It wasn't as clean as we'd like."
So when the McHenry tubes opened in 1985, the state decided to do an upgrade, investing in three Unimogs and attachments that allow them to not only wash tunnels but also mow grass and blow snow.
"We call them Swiss Army trucks," Allison said.
It doesn't take long for the tunnels to get dirty, said Martara Hannah, who oversees operations at the two facilities. The McHenry tunnel transports 3.3 million vehicles a month and the Harbor tunnel handles 2 million.
"You no sooner finish than it's time to start again," said Hannah, adding that it costs about $330,000 annually to clean the tunnels. The work is covered by tolls, not tax dollars.
The grime — exhaust, salt, oil and coal dust from the pile near the McHenry tunnel's northern entrance — really builds up during the winter, when washing is suspended, making April the cruelest month for authority workers.
The McHenry tunnel was considered an engineering marvel when it opened, the largest underwater highway tunnel on the Interstate 95 corridor. It is so large that the fresh-air duct that runs beneath the road surface was used to film a car chase scene for the 1998 film "Enemy of the State," starring Will Smith.
There's nothing speedy, however, about tunnel washing.
The workers go at the job methodically, washing the ceiling the first night and a wall on each of two subsequent nights before hand washing around lights, antennas, cameras and overhead signals.
The Unimogs line up like lumbering circus elephants, one behind the other, with two rinse trucks bringing up the rear. Each Unimog has a driver and brushman. The drivers maintain their position by aligning a red laser dot that the truck projects to a line on the wall or ceiling.
A boom on the first truck reaches from the ceiling to a point about halfway down the wall. The second truck covers the rest.
They move at a slow walk, stopping now and again to give a particularly nasty spot some extra attention. As spray fills the air, the 6-foot-long brushes dance a soapy rumba on the tiles, washing away grime — and the occasional game of tic-tac-toe left behind by idle motorists waiting out a traffic jam.
"We tell people to stay in their cars, but obviously they don't," said Kelly Melham, authority spokeswoman.
A geyser of clean water from the rinse truck chases the filth down storm drains that lead to a holding tank beneath the tunnel. A city plant treats the dirty water before releasing it. A four-hour cleaning shift uses about 20,000 gallons of water.
Sometimes an accident or breakdown forces the authority to halt tunnel washing and reopen the tube to alleviate traffic snarls.
"Then you'll see part of a wall clean and part of it dirty," Hannah said. "People notice. People talk."