Graffiti fighters from 52 cities are in San Francisco this week for the first conference of Zero Graffiti International. Hundreds of police officers, public works officials and others are sharing tips about an expensive problem.

Conference organizer Drew Lindner said graffiti abatement cost local governments $17 billion nationwide in 2009.

"It’s defacement, Lindner said.  "I mean some of it looks really nice. But one of the main things at the conference they’re talking about, graffiti on its own can be wonderful. It’s a beautiful piece of art -- but graffiti with permission. Graffiti without permission is vandalism."

The conference grew out of a website that helps people report graffiti. Lindner says people are fearful when they see graffiti and are often afraid to report it. But he says the best way to fight it is to get rid of it.

"It’s like a weed in your garden," he said. "You’ll see one tag, and if you don’t take care of it within a couple hours or a day, you’ll see another tag. And if you don’t take care of it, it snowballs."

The conference features effective anti-graffiti programs and exhibits of cleaners and graffiti-resistant wall coatings.