When filmmakers say they've been working on a project for 45 years, what they typically mean is that they thought about making a movie for 44 years and then finally got around to doing it.

But when Al Milgrom says he worked on the documentary "Singin' in the Grain" for 45 years, he really means it, and he has the proof: faded 16-millimeter film footage he shot in 1974.

Of course Milgrom, who is 96 — "I bill myself as the oldest emerging documentarian in the world, although I don't have much competition" — did lots of other things in those four-plus decades.

He founded what is now the Film Society of Minneapolis St. Paul, taught cinema at the University of Minnesota and launched the Minneapolis St. Paul International Film Festival, which kicked off Thursday with more than 250 movies, including Milgrom's.

"Singin' in the Grain," which sold out its Saturday premiere, follows a New Prague polka band through three generations, mapping the link between a culture and its music. The film has two more screenings during the 17-day-long festival.

"I don't know how much polka music people are willing to sit through." Milgrom said with a laugh a few days before the film's debut. "We never had a chance to run it past a test audience, so I guess we're going to find out."

Subtitled "A Minnesota Czech Story," the film's original focus was not polka history but rather the story of the Czech community that immigrated to Minnesota and still remains surprisingly tight. A big reason for that closeness, Milgrom discovered, was the music handed down from one generation to the next.

Milgrom's interest in the topic dates to his youth.

"I grew up in a Czech town, Pine City," he said of the town 65 miles north of the Twin Cities. "A lot of Czech immigrants moved there in the '20s and '30s following World War I. I grew up with a lot of Czech kids. I used to claim that I could even speak a little Czech when I was 10 years old. Czech was spoken in the kids' homes, and I picked it up there."

Pine City was far in his past, though, when he heard about a polka band playing a hoedown in New Prague. The bandleader had the same name as one of Milgrom's childhood friends, so he went to see if they were one and the same.

They weren't, but in the process, he met Eddie Shimota Sr., a second-generation polka musician who was grooming his young son, Eddie Jr., to follow in his concertina-playing footsteps. Milgrom pulled out his 16mm camera and started shooting. And 45 years later, he stopped.

Many things changed over those years, of course. The younger Shimota grew from a young boy to a middle-aged man. And the technology behind the film radically evolved, going from what was basically handheld home movie footage to digital cameras and drones.

Even then, it wasn't his idea to stop filming, Milgrom admitted.

"I had a hundred hours of footage, and everyone kept saying, 'You've got enough. Stop.' But the curse of being a documentarian is that you always want one more shot."

Dan Geiger, a Minneapolis-based film editor whose credits include "Fargo" and "Purple Rain," looked at all the footage "and didn't want any part of it," Milgrom said. "He said it was too chaotic."

But he agreed to tackle the job if Milgrom would quit adding to it. "He took 100 hours of film and turned it into 100 minutes," Milgrom said of Geiger, who wound up as co-director.

Milgrom also realized it was time to wrap up the project.

"I don't know how much longer I'll be around," he said. "I didn't want someone finding a hundred hours of footage in my basement and wondering what it is."

He's not being morbid. On the contrary, he has plans for the future. Lots of plans. "I have three more movies I want to make," he said. "I'm glad I finally got this out of the way."

Or, at least, mostly out of the way. He's proud of the final product, "but I still think of the other 99 hours" of footage that didn't make the final cut. "We had to pick a story line and stick to it, but there was some really good stuff in there" that got left out.

It's all been digitalized so "it would be easy to edit," he mused. Which means that someday, perhaps, he'll have one doozy of a director's cut.

Singin' in the Grain

April 6: St. Anthony Main Theatre, 115 SE. Main St., Mpls., 2 p.m.

April 17: St. Anthony Main Theatre, 4:15 p.m.

April 18: Marcus Rochester Cinema, 4340 Maine Av. SE., Rochester, 12:15 p.m.

Tickets: $8-$15, mspfilm.org.