How Do They Do That? Turn a Bus into a Billboard

Wouldn't your friends be amazed if you pulled up to school in a submarine? Or rode to soccer practice aboard a giant ham-and-cheese sandwich?

These days, city buses don't always look anything like they used to.

With the help of "bus wrap," these 40-foot-long, diesel-guzzling behemoths are being transformed into rolling billboards.

The wrap is a vinyl coating made by 3M that sticks to the outside of a bus. It was invented about six years ago.

The four-foot-wide strips of very sticky vinyl can turn a bus into a sunny beach in Hawaii or a showcase of Christmas artwork by children. If you live in a big city, perhaps you've already seen or even ridden on a bus decorated this way.

The special wrap used to coat the bus windows looks solid from the outside. But passengers can see out, even though people outside can't see in (unless it's night and the bus's interior lights are on). Window wrap is pierced with thousands of tiny holes - about 1,000 per square foot. Up close, it looks like the bottom of a colander, that thing you use to strain spaghetti. From inside a wrapped bus, everything outside looks darker. (The windshield of the bus is not coated.)

Before it goes on the bus, the computer-printed wrap looks and feels like a thin shower curtain.

Most buses are wrapped with ads. One cruise-ship line has wrapped a fleet of buses in big cities across the United States to look like miniature cruise ships. In New Orleans, a tour company wrapped a bus to look like a giant alligator with teeth more than two feet long.

"People love riding around in my huge alligator, and I love driving it," the driver of that bus says. "It's the most requested tour bus in the city."

One of the most popular buses in Los Angeles is a submarine bus. It was wrapped to promote the movie "The Hunt for Red October."

A giant ink-jet printer

Before a bus is wrapped, a small version of the art is scanned into a computer. Then an artist figures out how the image will look and fit on a bus.

A giant ink-jet color printer then spits out a full-sized paper copy of the the art. Each printout is four feet wide and 15 feet long! The printouts are used to "ink" the vinyl wrap.

"Inking" involves a giant machine that uses heat and pressure to transfer the image on the copy onto sheets of vinyl.

The inking machine puts two layers on the vinyl as it is inked: A clear layer on top protects the image, and a paper layer on the back is peeled off just before the wrap goes on the bus. The vinyl layer is sticky and has a heat-activated adhesive on it, too. The wrap is rolled up in a tube until it's ready to be installed.

It takes four people about 12 hours to wrap a bus.

First, the "wrap team" inspects the bus. There can't be any rust, loose paint, or dents. All those things would keep the wrap from sticking properly.

Next, the team carefully cleans the bus with special cleaners that remove all dirt and grease.

"A clean bus is the key to getting good adhesion of the wrap," says Shawn Comito, a Boston-based wrap installer.

How many vinyl printouts does it take to wrap a 40-foot bus?

Twenty-eight: 11 on each side, and six for the front and back. That's not counting the special vinyl for the windows. The wrap is hung lengthwise, like wallpaper. Installers start at the back.

"The back of a bus always poses the biggest challenge," Mr. Comito says. "It's tough to wrap around brake lights and air-intake vents."

What if it goes on crooked?

To wrap the sides, the crew starts in the middle and works its way out. One person stands on a wheeled scaffold and holds up the wrap while another uses a tape measure to align it according to the plan. If it goes on crooked, it can be carefully pulled off and reapplied.

"We try to get it right the first time because the adhesive glue sticks better, and that makes the vinyl stay on longer," Comito says.

When the wrap lines up properly and is stuck down, the crew uses very sharp razor blades to carefully cut around bumpers, lights, vents, and other parts of the bus that can't be covered. The special window wrap is applied separately.

With squeegees and rollers, installers then press the vinyl against the bus so it will stick well. Any air bubbles are popped with a pin and flattened.

Now the bus is ready to be "cooked." The crew uses heat guns that look like big hair dryers, but they get much hotter. The heat activates another adhesive on the vinyl that makes it stick tight.

Once the wrap is cool, the bus is ready to roll. The wrap lasts five or six years, according to 3M. If it's ripped or damaged, it can't be fixed unless the whole sheet is replaced. More heat guns are used to remove the wrap. But the biggest threat to the wrap is sunlight. It fades the ink.