Friedensreich Hundertwasser, Austria's most celebrated artist and one of the most successful painter-print-makers in the world, stood up in his Washington hotel room and beamed at Arlington co-op owner Mireille Alberti. w

"For me you are a hero," the millionaire artist said. While abroad, he had been sent a recent newspaper account of how Alberti was taken to court for installing a bay window without her co-op's permission. He immediately phoned, offering financial and moral support. She accepted the latter.

So when he arrived in Washington from Vienna this week to donate a poster to Ralph Nader's energy project -- prompting Mayor Barry to declare today Hundertwasser Day -- Alberti was one of the first people he wanted to see.

To record his meeting with Alberti on Sunday, Hundertwasser asked his business manager to take a picture of the pair. But first the balding, bearded ascetic added a peaked knit cap to the odd assortment of clothes he was wearing: homespun striped cotton trousers, baggy black velvet jacket and Earth Shoes.

Alberti's plight is "a great scandal," said Hundertwasser, 52, whose many manifestoes include one on the subject of "Fensterrecht" -- the right to one's window. "If somebody lives in one of these human silos and revolts, he should have all assistance from the state and at least moral support from his fellow citizens,"

"Perhaps I can to to jail for her," he said, his blue eyes lighting up at the thought of another "manifestation" -- his word for his often outlandish behavior on behalf of environmental causes.

On one such occasion he addressed a Vienna audience nude to protest the "inhumanity" of a new building. "I live my revolution," he said. "Every man has three skins: his epidermis, his clothes and his house. To deprive someone of doing what he wants with any of these is equivalent to putting him in a concentration camp." Forty members of his family died in Nazi concentration camps in Austria during World War II. He and his mother survived the war in Vienna.

Hundertwasser is a one-man art industry. He has made a considerable fortune through his paintings (which sell for $35,000 to $140,000) and woodcuts, etchings and silkscreens (which sell for $500 to $3,000) -- not to mention the books and scarves and reproduction rights.

But he is also a cult figure, hermetic, environmentalist and proponent of sod roofs and humus toilets, and he came to Washington to meet one of his greatest heroes, Ralph Nader. Tonight, at a reception at the Phillips Collection, he will donate an entire new edition of 10,000 Hundertwasser posters -- to be sold at $40 each -- to benefit Nader's anit-nuclear Critical Mass Energy Project.

In the past, the best of his posters have often increased dramatically in price after the initial sale. One commissioned by the German government for the 1972 Munich Olympic Games, originally issued at $15, now trades at $750. tHe recently donated another poster edition to the United Nations, and his postage stamps for Senegal, Austria and Cuba are cherished collector's items.

Sunday afternoon, clad in full-length blue down coat, the artist was looking forward to the Phillips event as he walked the bleak expanse of Judiciary Square at 5th and D streets NW, where 13 new trees, his unsolicited gift to the city, will be dedicated at 10 this morning. "I'm always curious to meet people like Nader who do something good," he said while examining the site. "He comes from a different starting point, but ends up with the same conclusions I have reached."

Hundertwasser claims 100 kilometers per liter for his motor-driven bicycle, and better than that for his colorful sailboat, Regentag (rainy day). He lives on the boat when not growing trees and moss in, on and around his various homes in New Zealand, Austria, France and Italy. He has been married twice and divorced twice, but maintains that his love for mold and humus in the house had nothing to do with either breakup.

The trees cost Hundertwasser $8,200, a figure he heard from his Washington dealer, Manfred Baumgartner, who arranged the gift. "Money is for me a side-product of creativity," Hundertwasser said. "I invest it in the ecology, in tree planting and in livable architecture. I'd like to have more. People who build atomic power plants are an international mafia. I can never have enough to fight them."

He talked little about his art, but when pressed, produced a watercolor he had made during the flight from Vienna -- a typically joyful composition of brightly colored spiral forms and schematized automobiles -- which he plans to finish in oil, perhaps with his characteristic metallic embossing. "I work a lot in planes these days," he chuckled. He will soon be on another plane back to Vienna, where he is currently engaged in a major public housing project -- a building with grass on the roof and trees growing out of apartment windows. "Environmentally, trees make far better tenants than people," he said. Each window in the proposed building will be different. "Needless to say, they can all be changed," he added.

The only book in Hundertwasser's hotel room was "The Guinness Book of World Records," which he had picked up in the London airport. He hopes eventually to be listed in it as "the longest-running one-man show in history." eA retrospective exhibition of his work has been touring world capitals and outposts for five years -- at his expense.

His only Washington show, outside of those at Baumgartner, was at the Phillips Collection in 1969. Though widely known in Europe and among a devoted band of American collectors, he is less well known here, and there are no works by Hundertwasser in the permanent collections of Washington's museums.

He will visit no museums while here. "What you do with nature is always better than what you do with your intellect," he said. "These manifestations are closer to my heart than my art. An artist must fight, not just sit on his laurels."