For nearly 60 years from the late 1940s the artist and teacher Frank Mason challenged art restorations around the world, including at the Metropolitan Museum of Art in New York and, later, at the Vatican Museums in Rome. Many artists before him, such as Goya, David, Delacroix and Degas, had attacked what they felt were careless or overzealous restorations, but the longevity of Mason’s campaigning was unprecedented and he lived to see much of it gain art-critical vindication if not full political victory. Had he not been so traditionalist an artist he might perhaps have achieved greater institutional influence. But, then, had he been a more fashionably paid-up Modernist unversed in traditional art’s mysteries, he would less effectively have pinpointed the errors that restorers make. A fractionally damaged glaze in an Old Master painting was as glaringly obvious to Mason as a tear would be to others on a Mondrian.

Frank Herbert Mason was born in Cleveland, Ohio, and moved with his family at the onset of the Depression to New York City, where he became one of the first students to enrol in Mayor LaGuardia’s experimental Music and Art High School. Not long afterwards, he found his calling at the age of 16 at the Art Students League of New York within the classroom of Frank Vincent Dumond — whose students included Georgia O’Keeffe, Norman Rockwell and John Marin and whose own student days at the Académie Julian, Paris, had brought contacts with fellow Americans Whistler and Sargent. Under Dumond’s tutelage (which had begun at the Art Students League in 1892), Mason rapidly evolved as a traditional artist. He became the youngest instructor at the league when he took over Dumond’s class upon his death in 1951.

For well over half a century Mason, like his master before him, elevated demonstrations of painterly technique over artistic theories. Students were encouraged to paint the illusion of subjects physically located in space and atmosphere, as revealed through the medium of light.

From his earliest years at the league, Mason investigated techniques and mediums used by artists in the 16th and 17th centuries. In close collaboration with Jacques Maroger, a painter who from 1930 to 1939 had been the technical director of the Louvre Museum’s laboratory, he sought to rediscover and apply lost technical working methods. Such investigations further alerted Mason to the nature of injuries being unwittingly inflicted on paintings in the postwar “modernising” art restoration frenzy.

An early notable battle was over the renowned Rembrandt collection at the Metropolitan Museum of Art in New York. On February 17, 1976, after many years of unavailing complaint, he led a large group of artists and students from the Art Students League on 57th Street to demonstrate outside the museum on Fifth Avenue. A press release announced: “We are here to protest the destruction of our national heritage. In the last 25 years paintings have been literally skinned through over-harsh methods of conservation. The result is damaged pictures that look piecemeal and no longer represent their period but rather seem to have been done in the modern method.”

A year after the demonstration, the Canadian author and collector of Old Master paintings George Encil contacted Mason to say that he and other private collectors agreed that most museum cleanings in the United States were indeed injurious. Encil invited — and paid for — Mason to spend a week studying the more judicious cleaning methods then employed in Vienna at the Kunsthistorisches Museum and other famous collections. In June 2000 Mason’s objections found further support when Alexander Eliot, a former arts editor of Time magazine, said that in the early 1950s the director of the Metropolitan Museum then, Francis Henry Taylor, had confessed to him: “Oh, we ruined the Rembrandts ourselves.” In the ArtWatch UK Journal of the autumn of 2001 Mason recalled his dismay on first seeing several recently restored Rembrandts on a joint visit with Dumond to the Met in 1947. “I was shocked to recognise definite alterations to these familiar and beloved works,” he said.

As well as leading such protests, Mason also executed many artistic commissions. One of the most important came in 1962, when the Sovereign Order of the Knights of Malta commissioned him to paint a series of eight large pictures depicting the life of St Antony of Padua for the 11th-century church of San Giovanni de Malta, in Venice. These 8ft by 5ft canvases were exhibited to international fanfare at French & Company in New York before being shipped off to Venice. Mason was awarded the Cross of Merit, Prima Classe.

Mason established an international reputation as a portraitist in the 1960s, taking time off at the league to complete commissions ranging from Prince Giacomo Colonna of Venice to the New York Governor Averell Harriman (one of Mason’s portraits hangs in the US Embassy in London where Harriman served as ambassador in 1946-47). Mason also executed murals for King Faisal for the Saudi Arabia Naval Academy. Mason’s anti-restoration exertions culminated in the late 1980s when, with the support of his fellow painter Pietro Annigoni (who in 1970 had painted the word “murderers” in capital letters on the doors of the National Gallery in London, having earlier, in July 1956, initiated a heated correspondence in The Times on the subject of the gallery’s approach to restoration), he challenged the Vatican’s cleaning of Michelangelo’s Sistine Chapel frescoes. It was then that Mason joined forces with the late Professor James Beck, of Columbia University, Alexander Eliot, the British artist and illustrator Michael Daley, and the Russian-born cultural historian Arkadi Nebolsine. Although their combined campaigning failed to halt the Michelangelo restorations, it drew worldwide attention to the cleaning methods then being applied to Italy’s artistic patrimony. Belatedly, those protests about the Sistine Chapel attracted support from scholars (such as Charles Hope, the director of the Warburg Institute) who had initially been enthusiastically supportive of the restoration. The collaborations led to the formation of ArtWatch International in 1992, and of ArtWatch UK in 1995, watchdog organisations specifically dedicated to protecting the integrity and the dignity of works of art and architecture from injurious or falsifying restorations.

Mason’s views on restoration testified to a rare artistic insight and humility: “As artists we know that a fine oil painting does not possess a hard impermeable surface, but that it is comprised of layers of ground pigments, suspended in elastic films of various oils and varnishes which are superimposed, interwoven, and melting into each other in a way which not even the artist can accurately map. In spite of what conservators would have us believe, science cannot objectively scrutinise a painting and accurately enumerate all of its components in any meaningful way; a plain chemical analysis is too crude a tool to measure the ineffable.”

Little surprise, perhaps, that his instructions at the Art Students League should have captivated and inspired generations of students over the course of 57 years.

Mason is survived by his wife, Anne, and by his two sons.

Frank Mason, artist and teacher, was born in 1921. He died on June 16, 2009, aged 88