Michael Botwinick, the departing director of the Corcoran Gallery of Art, has taken a new job, but not in a museum. On Monday he will begin working for a private art-dealing firm that is largely owned by Dr. Armand Hammer, the billionaire collector and a Corcoran trustee.

The firm -- Knoedler-Modarco -- is the parent company that runs Hammer's two commercial New York galleries, Hammer Galleries and M. Knoedler & Co.

As a senior vice president, Botwinick said he will be "primarily responsible for revitalizing the historical paintings division of Knoedler & Co."

Knoedler's, whose New York branch was founded in 1846, once was widely known for its holdings of old masters. But in recent years, that aspect of its business has declined.

Botwinick, 43, who became the Corcoran's director in 1983, announced his resignation last Christmas Eve. He has not been replaced.

His museum career is probably over. The boundary between museums and commercial art dealing is a sort of one-way membrane. Commercial galleries have frequently hired museum specialists, but good museums almost never hire dealers.

"One reason it is a one-way membrane," Botwinick said yesterday, "is that no one has ever wanted to come back." He acknowledges he is venturing out on a new career. "I think of this as something brand-new," he said. He will continue living in Washington and commute to New York.

"I began working for museums 18 years ago this month," he added. "I have worked for four of the greatest in the country {the Metropolitan Museum of Art, the Philadelphia Museum of Art, the Brooklyn Museum and the Corcoran}. It's been a long career. But in this life we get to have lots of different careers. I'm enormously excited about what's ahead."

Armand Hammer has been selling art for nearly 60 years. The Hammer Galleries, which he founded with his brother Victor in 1928, now serves a clientele that some describe as middle-brow. Grandma Moses and LeRoy Neiman are among the painters whose works it represents.

Knoedler's -- which Hammer Galleries acquired for $3 million in 1971 -- sells first-rate 20th-century art. Its director, Lawrence Rubin, is a major player in the commercial New York art scene. His gallery represents Frank Stella, Richard Diebenkorn, Robert Motherwell, Howard Hodgkin, Nancy Graves, Jules Olitski and the estate of sculptor David Smith.

Hammer bought Knoedler's after learning "that while Knoedler's carried its inventory on its books at cost, as is customary, any item which it hadn't sold in 10 years was written off as worthless." In his autobiography he wrote that that "inventory alone was worth many times the asking price for the whole company."

Since 1971, however, that inventory has been largely depleted. Rebuilding it will be one of Botwinick's responsibilities.

At the Corcoran, Botwinick was paid $98,000 a year. Though he declined to state his new salary, he did say "money was not an issue."