"Spray paint is a very unpredictable medium," says Eaton, who goes through about 40 cans a day as he works to bring Frankenstein's monster and his Bride to life alongside Dracula, the Wolfman and the Creature from the Black Lagoon, originally created by the long-overlooked designer Milicent Patrick (the subject of a new book coming out March 5) for the 1953 film, whose star Julie Adams died on Feb. 3.

Universal's monsters "were the outsiders, outcasts, rebels and outlaws of their time," says film chairman Donna Langley. It's fitting that a graffiti artist is re-creating them, she adds, "because graffiti artists are considered the rebel outlaws of the art world."

Before starting the project in mid-January, Eaton went through Universal's North Hollywood archives to study props and posters from the 1930s and beyond, taking particular inspiration from European posters whose art sometimes differed greatly from the films. One of the mural's Frankensteins has laser eyes, an homage to an early poster that touted Bela Lugosi, rather than the film's eventual star Boris Karloff. "All of these monster characters are open for interpretation," says Jeff Pirtle, NBCUniversal archives and collections director.

Holly Goline, the Universal film exec who oversaw the mural, considered 12 different locations for the project before selecting the wall of a massive soundstage on the northwest corner of the Universal lot. "If I'm only going to get one wall, I want the biggest one," says Goline, who hopes Eaton will return to do more murals of characters like the Invisible Man.

Eaton spent a month working sunrise to sunset, starting in mid-January, but an unusually rainy period in L.A. put him behind schedule. In early February, the studio rigged up lights (see time-lapse video below) so he could paint at night as he raced to finish before jetting off to New York for a different building-sized mural.

On a visit to the work in progress, which can be viewed by some visitors on tram tours, THR overheard security guards debating its merits, and Eaton gets stopped by studio employees who confess they used to do (unsanctioned) graffiti in their younger days. Universal is hosting a food truck lunch event to showcase the mural to employees today.

The project's visibility is exhilarating for the 40-year-old artist, who's had high-pressure gigs before: He created four posters for Barack Obama's campaign in 2008. In politics, Eaton notes, half the audience will hate your work. But the monsters? They're for everyone. That's led him to continuously redo aspects of his work, trying to make each monster equally potent. "I'm working my butt off for this," says Eaton, "because I love it so much."