The nicest thing about "Jacob Kainen: A Video Portrait," the lightweight documentary that airs tonight at 9:30 on Channel 26, is its likable and thoughtful star, the Washington artist Jacob Kainen.

Kainen is a lovely guy. This 30-minute show claps him on the back, but performs no other service. It isn't thrilling entertainment. As a piece of scholarship it is next to useless. It's too shallow and too sweet.

This insubstantial show is the first television program produced by the National Museum of American Art, where Kainen used to work. It was directed by Alison Abelson of the museum's education department. A Kenneth Clark she's not.

She seems to like her subject. So does everybody else who has met him on the art scene. He is a connoisseur, a scholar, a friend to art and artists, and a dedicated painter. Halfway through the program, an unidentified voice informs the viewing public that Kainen is "one of the half-dozen best painters in this country." The voice, as some will recognize, belongs to Harry Rand, the museum's curator of 20th-century art. Does he really believe that? If so, he ought to stand up and substantiate his claim. That's the trouble with this awe-struck show. Though made by a museum, it is as praise-filled as an ad.

Kainen, there's no doubt, has had an interesting career. He hung out in New York with Arshile Gorky, Stuart Davis, John Graham and other well-known artists before he moved to Washington in 1942. During the Depression, while his poorer friends were forced to live on ketchup, Kainen sometimes witnessed whole city blocks of families being evicted all at once. "Being a little on the radical side in those days," as Kainen says in the show, he made socially conscious prints for the Works Progress Administration. When he first came to Washington, this city was so square that the critic for The Evening Star objected to the showing of a painting by Cezanne at the National Gallery of Art.

Kainen, more than once, has significantly altered the style of his art. He made representational pictures in the '30s and the '40s, abstract paintings in the '50s, and in the 1960s, when doing so took guts, returned to the figure. Now he's making abstract pictures once again.

We are told his story vaguely. We never learn how old he is (he was born in 1909). We see many of his pictures, they fly by on the screen, but we are never told their titles or their dates. And there is no way the beauty of their subtle, layered colors can be fully seen on the television screen.

Television, here as elsewhere, almost always makes painting seem pretentious, unimportant or boring. Charles Eldredge, the new director of the National Museum of American Art, says that his museum intends to produce more TV shows.

It is nice that the museum is nice to Jacob Kainen, but if its future programs are as thin as this one, it might as well devote its limited resources to hanging paintings on its walls.