De Ruth's painting used to occupy him eight to 15 hours a day. Now he is down to about seven hours. He works very rapidly, with intense concentration. "I don't paint after the afternoon," he explains, "except sometimes sketching at night. You exhaust your juices by the time evening comes along." One person he used to sketch after hours was actress Karen Black, who lived in West 68th Street just across from his apartment. Says De Ruth: "she would sit in the in the windowsill in her bra and slip. Then one day I called over to her, 'Would you like to get paid for this?' She rushed inside to get her glasses, and looked over at me, very surprised. She became my model for some time." For a woman to be an ideal nude model, said De Ruth, "she should be gentle, as intelligent as possible, considerate, and somebody in the arts, or with the sensitivity of an artist. And she must be physically attractive." How do the women who pose fully dressed for commissioned portraits compare to the professional nude models? "They work better than my models usually," says the artist, who has painted Ethel Kennedy, Eleanor McGovern, and the late Martha Mitchell for _Time_. "They're much more concerned to participate. I don't think it's necessarily something to do with vanity. It's much more curiosity. Because we never really know until the day we die what we look like. Because we vary so much from one time to another." Ironically, Martha Mitchell -- wife of President Nixon's infamous attorney general, John Mitchell -- posed for De Ruth inside the Watergate Building during the height of her fame. "She had a certain peasant charm -- a charm of her own," he recalls. A man who craves variety, De Ruth has for many years spent his summers at a studio in Massachusetts. This past summer he began to teach painting in New Mexico -- something he has wanted to try for a long time. A passionate skier, he travels to Austria each winter to pursue the sport that he learned as a child, then gave up until his mid-40s. His other after-work activities? "I love to be in the company of women," says the artist with a radiant smile, adding that he prefers their company when he's not painting them. The East Side, according to the artist, is "a city in itself. There's a sterility over there, at least for me. I just can't see myself without this mixture that the West Side is." De Ruth has been going to the same Chinese laundry for 28 years -- Jack's on Columbus Avenue. Another business he has patronized all that time is Schneider's Art Supplies at 75th Street and Columbus. As the interview comes to a close, I ask De Ruth what advice he would give to an aspiring young artist. "Never be discouraged by anyone or anything," he says. Then, to balance his remarks, he relates an anecdote about an art student who asked Degas what he could do to help the world of art. Replied Degas: "Stop painting." ******** WESTSIDER MIGNON DUNN The Met's super mezzo 3-8-80