MADRID — Manolo Osuna lacks a formal art education, but he has spent years roaming the galleries of the Prado Museum as a guard and leader of a seven-person moving brigade that hefts national treasures by Spanish masters like Velázquez and Goya around the building.

With that homegrown background, Mr. Osuna, 56, has emerged from an invisible role at the museum to become an unlikely art critic in an Instagram video series that has become a hit. The videos, shot with a cellphone and selfie stick, have attracted a growing international following of up to nearly 100,000 daily viewers, who are fascinated by the slow-paced, decidedly un-Hollywood view of the museum, where a truly special experience is watching paint dry.

Every weekday, in the low hum of voices before the Prado, Spain’s national museum, opens, curatorial superstars and uniformed guards in red scarves are given a precious 10 minutes to talk. They focus on the works that are their familiar neighbors: the flirty 19th-century aristocrat in pale, green satin and pearls, or the Virgin Mary swooning below a crucified Christ.