It happens overnight: A plywood construction barrier goes up and is promptly plastered with ads that turn half a block into a billboard for the latest romantic comedy or pop album.

There’s no question that wheat-pasted ads are common in the Mission, but are they legal?

In a word, no, but there are enough loopholes to make the option attractive and doable for anyone who doesn’t make a habit of it.

For some, wheat-pasting is art.

Galeria de la Raza has used wheat-pasting and billboards as a way to promote its exhibits. Adriana Grino, the gallery’s curatorial and special programs manager, said that they’ve asked artists to add graffiti art to their billboard.

A few months ago, the gallery asked artist Jesse Hernandez to spray paint on its billboard on Bryant Street. That was at the Galeria’s request, but most often artists act without an invitation.

“It’s a form for artists to get their art out there,” said Grino. “It’s just an outlet for public dialogue.”

The city prohibits what’s known as general advertising — signs that tout anything that isn’t for sale inside the establishment where they are posted. That means posting on construction barriers is illegal, and corner stores can advertise beer and soda if they sell it, but not movie tickets or CDs.

The signs don’t sit well with Milo Hanke, past president of San Francisco Beautiful, a nonprofit that aims to protect and enhance the city’s beauty and livability. To him, the signs are visual pollution that degrade neighborhoods.

“These ads rob communities of their distinctiveness and rob us of our sense of place,” Hanke said, adding that “rogue” and “parasitic” companies target poor neighborhoods to blanket with advertising. “You see very few billboards in Pacific Heights.”

“It’s up to the viewer whether it’s art or not,” Grino said.

So why are these signs everywhere, and what about those signs that advertise mayoral candidates, websites, free events?

A lawsuit between the city and an advertising company, and the difficulty of tracking down those responsible for the signs, means that many of these illegal ads stay up — at least for long enough to get the word out about whatever is being sold.

The first thing to understand is that enforcement of the city’s general advertising ban is split between the Department of Public Works (DPW) and the Planning Department. The Planning Department is responsible for keeping private property free of illegal advertising, while DPW is charged with keeping the public right-of-way — including temporary construction barriers — free of flyers.

DPW has issued 42 citations this fiscal year to companies that posted illegal ads on public property. While the department was unable to say exactly how much that translates to in fines, Gloria Chan, a DPW spokeswoman, said that fines can range from $50 to $500, depending on the size of the sign, and an additional charge of 50 percent of the fine is tacked on for inspection and removal.

Chan said that DPW’s efforts to crack down on illegal posting are often stymied by a lack of identifying information on the ads. “It is challenging for the department to identify the culprit of illegal ads,” she wrote in an e-mail.

The Planning Department faces a similar problem when it comes to enforcing the general advertising ban on private property, said Jonathan Purvis, the staffer responsible for policing general advertising on private property.

So they go straight to the property owners, not the companies that put the ads up. “The companies are hard to track down,” Purvis said, “and many of them aren’t operating aboveboard.”

Multiple calls to a number of companies advertising wildposting services were not returned.

The Planning Department has issued $87,000 in citations for illegal general advertising on private property in the last year, and $420,000 in the last five years, Purvis said.

A crackdown on illegal billboards conducted by the department over the last three years has resulted in the removal of almost 800 ads.

Still, many buildings, including a cluster around the intersection of 24th and Folsom streets, have billboard-sized frames on their facades that are usually blanketed with wheat-pasted ads for movies and other products.

So why hasn’t the city taken them down? A close look at the frame yields fine print stating that the posters are part of a contest sponsored by Concert Promotions, LLC, a Culver City-based operation, and that would-be contest participants should inquire within for details. As Mission Loc@l reported in July, the company and the city are involved in a lawsuit over whether or not the ads violate the city’s ban on general advertising. The city claims they do, but the company counters that the ads are advertising something available in the store — a contest. The city can’t take enforcement action against the company until the lawsuit is resolved.

Fines for building owners start at $100 per day for small signs and reach $2,500 per day for the largest billboards. Purvis said that the illegal signs in the Mission are small enough that none would rack up more than $1,000 per day — still a significant sum.

The first time the Planning Department sees a violation, it issues a citation, giving the building owner 30 days to remove the post before incurring a fine.

That’s a loophole advertisers use to get the word out about their products, said Jim Chappell, interim director of San Francisco Beautiful. “The city gives people 30 days to take down illegal ads, but they’re movie ads a lot of times, so they don’t care, because by 30 days they’re stale anyway.”

But the Planning Department has little tolerance for repeat violators, Purvis said. If illegal signs are found in the same spot more than once, the property owner has only three days to remove the ads. About 25 percent of the fines collected are for repeat violations.

With the exception of the billboards the city can’t touch, the situation is under control, Purvis said.

“It’s easy to see the violations. They’re right there, because that’s the whole idea of advertising — to have maximum visibility.”

Still, Hanke said, the fact that enforcement of the ban on general advertising is divided between two city entities is diminishing its potential effect. He hopes the issue will gain profile during the upcoming mayoral campaign, and wants to see the laws streamlined and the fines and enforcement increased.

The problem is yet another blow to the communities that already struggle the most economically, Hanke said. “The more billboards that go up in the Mission, the less I see the Mission.”

The Planning Department maintains a map of places in the city where general advertising is allowed. Any ads in areas that are not on the map are most likely illegal. Citizens who want to report a sign they believe to be illegal should call Purvis directly at 415-558-6354, or the advertising sign compliance hotline at 415-575-6863. But Purvis warns that if the signs are more than a year old, the city is probably aware of them but not able to take them down because of pending litigation.

Hélène Goupil contributed to this article.