Robert Cenedella, 76, is known for his aggressive and politically charged work,which aims to thumb a nose at the establishment, such as his painting of Santa on a cross. “If you compromise with your art, why be an artist?” he says. (Robert Cenedella/LeCirque/CAVU Pictures)

All his life, Robert Cenedella has never quite fit in. He was an outsider at home, where he discovered, at 6 years old, that he was illegitimate. School was a nightmare, so he dropped out of high school. Later, when he found his calling — painting — the art world didn’t exactly open its doors to him.

And that was long before he painted a portrait of Santa Claus nailed to a cross.

Frumpy yet jovial and with a grandfatherly white beard, the 76-year-old Cenedella appears not to have lost that youthful spunk in Victor Kanefsky’s documentary “Art Bastard,” which zips through the artist’s lifelong war on the art establishment.

Cenedella first made his mark by selling an alternative to the popular “I like Elvis” buttons of the 1950s. His “I like Ludwig” (as in Beethoven) version earned Cenedella enough money to enroll at the Art Students League of New York, where he studied under German painter George Grosz while still in his teens. Like Grosz, Cenedella excelled at exaggerated portraiture, though his scenes had their own prickly energy.

His cityscapes tend to be filled with colorful characters engaged in aggression: a nun yelling at a truck driver; an elderly man groping a woman (who prepares to retaliate with an umbrella); a little old lady in red heels punching a man in the face. In an inspired move, the film adds sound effects — honking cars and blaring sirens — while panning across the pictures.

“Presence of Man” by Robert Cenedella. (Robert Cenedella/Presence of Man/CAVU Pictures)

“Second Avenue” by Robert Cenedella. (Robert Cenedella/Second Avenue/CAVU Pictures)

Cenedella is also known for his politically charged work, whether painting police officers with dog heads or making a series of dart boards with the face of Richard Nixon on them.

Although his art was attention-grabbing, the elite didn’t embrace him. Just as Cenedella was getting his start, abstract expressionism and pop art were all the rage. Paintings with a point of view had fallen out of favor.

Cenedella’s way of thumbing his nose at the establishment was a style he called “Yes Art.” His answer to Warhol’s Campbell’s soup cans, for instance, was to paint Heinz soup cans. But the joke was lost on many: The show was a hit, and fans urged Cenedella to keep working in the pop-art vein.

That was never going to happen. Cenedella wanted to document the vibrant life of the city. “You can bastardize everything else in your life,” he says, “but if you compromise with your art, why be an artist?”

The documentary is a compelling indictment of the way commerce drives the art market. But the movie’s methodology is hit-or-miss, jumping from one interview to another, to jarring effect. When Cenedella’s sister reminisces about their childhood, she addresses her brother, rather than the camera, which feels stilted and strange.

The overall look of the film is closer to public-access television than polished documentary. Then again, such an approach may be fitting. Cenedella is rough around the edges, yet he remains fascinating.

Unrated. At the Angelika Pop-Up at Union Market. Contains nudity and strong language. 82 minutes.