Now 25 years after his death it turned out he was right - about one piece by Vincent van Gogh.
The painting, "Le Blute-Fin Mill," goes on public display Wednesday in the small Museum de Fundatie in the central Dutch town of Zwolle.
Louis van Tilborgh, curator of research at the Van Gogh Museum in Amsterdam, said the painting was unusual for the 19th century impressionist, depicting large human figures in a landscape. The painting shows Parisians climbing wooden stairs to a windmill in the Montmartre district.
But the work was typical of Van Gogh's at that time in other ways, with its bright colors lathered roughly on the canvas. Van Tilborgh said it was painted in 1886 when the artist was living in Paris. The canvas bore the stamp of an art store he was known to frequent, and used pigments that were common in other works, van Tilborgh said.
The painting "adds to his oeuvre," the curator told The Associated Press. "You can link it to certain works of Van Gogh in that period, but not that many of them," he said.
It is the first Van Gogh to be authenticated since 1995 and the sixth to be added to the confirmed list of the artist's paintings since the latest edition of the standard catalog was published in 1970, van Tilborgh said.
Van Gogh painted about 900 works in his brief career. Afflicted by mental illness, he died of a self-inflicted wound in 1890 at age 37.
Hannema bought the painting in 1975 from an antique and art dealer in Paris who did not believe it was of much value. But the Dutch collector did. He paid 5,000 Dutch guilders for this and another unknown work, or about $2,700, but immediately insured the painting for 16 times what he paid.
Hannema touted the painting with "absolute certainty" as a Van Gogh, but no one was listening. He had been discredited since he bought a Vermeer in 1937 that later was shown to be a forgery.
Hannema became director of the respected Boijmans Museum in Rotterdam in 1921 at age 26. Born to a wealthy art-collecting family, he was talented, successful, good looking and supremely confident in his judgment of art, said Ralph Keuning, the director of Museum de Fundatie.
During the Nazi occupation of the Netherlands he was given responsibility for all the museums in the country. After the war in 1945 he was arrested and stood trial for collaboration, but he was never convicted and was released from internment two years later.
He continued to add to his own collection, seeking out high quality work by lesser known artists and always looking for unattributed works of masters. He was mistaken nearly all the time.
"He was the laughing stock of the art world," van Tilborgh said. "His tragedy was that he was always thinking in terms of the big names."
In 1958 Hannema created an institute for his collection and was allowed to live in Nijenhuis Castle in the village of Heino on condition that he allow public access to the works, which included many fine classical and modern pieces. Some were on permanent display in two small buildings on the grounds, and he conducted tours by appointment of his home until his death in 1984.
Keuning said Le Blute-Fin Mill was not prominently displayed during Hannema's lifetime. "He was more obsessed by his Vermeers," which he believed to be authentic.
The collection formed the bulk of the Museum de Fundatie, one of the smaller institutions in this museum-rich country. It acquired another palace in the nearby town of Zwolle in 2005. More than half of the 7,000 items in its possession comes from Hannema's collection, said museum spokesman Koen Schuurhuis.
The museum had sought once before, in 1993, to have experts authenticate Le Blute-Fin Mill to prepare for an exhibition, Schuurhuis said. But the Amsterdam experts had no time, and the painting went on display as a work that Hannema "claimed" was a Van Gogh.