VATICAN CITY, APRIL 3 -- In the five centuries or so since Michelangelo transformed it into a Renaissance masterpiece, there's little the Sistine Chapel hasn't seen -- or endured.

Pigeons, insects and moralists have left their mark. So have old-time restorers wielding Greek wine, crusts of stale bread and animal glue. There have been explosions and leaks. Red-hatted cardinals choosing a new pontiff by the light of sooty torches and oil lamps. Zillions of perspiring tourists churning up dust and moisture.

Most recently, the chapel of Roman Catholic popes has found itself at the epicenter of a wave of culture shock as Vatican restorers rinsed away layers of dirt to reveal electric colors spread exuberantly across Michelangelo's magnificent frescoed vault -- shattering traditional perceptions of the artist and igniting a firestorm of controversy.

Still acutely aware of the criticism, Vatican restorers today concluded a meeting of 60 international art experts called to evaluate the nearly 10 years of work on the Sistine ceiling and to confer on how to handle the final phase, the "Last Judgment," inaugurated this week.

They have also opened a new show at the Vatican to document the restoration and, they hope, to silence art world opponents who claim they're "killing" Michelangelo's epic work.

It's unlikely the sound and fury will stop here.

A new debate has already erupted on preserving this latest restoration from today's environmental threats. There is the "delicatissimo" issue of how to handle loincloths and coverings added to naked figures in the "Last Judgment" to make them less risque. They were added over the centuries on the orders of conservative papal officials.

And then, restorers told a news conference today, there's the prospect that the contrast between the now dark and gloomy "Last Judgment" and its cleaned-up colors "will be even more astonishing" than the unveiled ceiling.

That's life, the restorers sigh, when you choose to work on one of the most celebrated icons of Western art.

"There's no other comparable case because there's nothing comparable to Michelangelo and his ceiling," said New York University Prof. Kathleen Weil-Garris Brandt, a consultant for Renaissance art at the Vatican Museums, in her Vatican office. "All of us," she says, were attached to the delicate pastels, grays and shadowing, the sober depiction of Genesis that epitomized the Sistine before its latest incarnation. Then suddenly, from under centuries of accumulated grime, sprang figures clothed in a brilliant palette of apple greens, cherry reds and banana yellows that seemed -- to modern eyes anyway -- to bespeak more passion than piety.

Badly punctured was the theory that Michelangelo, better known as a sculptor, was sparing in his use of color to preserve the "purity" of line and form used in his statues.

"It was a true cultural shock," Brandt says. "Now we in the profession have to rethink everything from the ground up ... we have to get to know him again as a painter."

Although the overwhelming majority of art specialists who have visited the restoration-in-progress have approved, artists still write to the Vatican and Italian newspapers pleading for an end to the "desecration." The project's most influential critic, Prof. James Beck of Columbia University, regularly petitions Pope John Paul II to halt what he calls "an artistic Chernobyl."

Beck, who climbed the scaffolding once to view the restoration in 1987, has charged that restorers have destroyed Michelangelo's finishing touches, or l'ultima mano. There's also the question of whether the artist himself put on a layer of glue to add a mysterious wash of shadow and even out the overall tone, though members of the restoration team say there's no evidence he did.

And Vatican restorers hotly deny they have destroyed any of Michelangelo's work, saying they were able to isolate his own "finishing touches" and distinguish them from subsequent alterations through pigment tests.

"We removed dirt. We didn't remove Michelangelo," insists Fabrizio Mancinelli, the project's director.

Although Michelangelo trained in painting as well as sculpture at the Florentine school of Ghirlandaio, he wasn't noted as a master of fresco technique. Certainly he wasn't comfortable with the task when prevailed upon by Pope Julius II in 1508 to tackle the formidable vault.

In letters and poems reproduced at the Vatican exhibit, he complains of 16-hour days in which he worked "beard to the ceiling," often on his tiptoes, in the unfamiliar and demanding medium that requires tremendous finesse and speed to finish the job before the wet plaster dries.

"I don't feel I'm in the right place, nor am I a painter," he wrote at the time.

This restoration indicates his confidence grew as he worked, to the point that he painted some of the smaller sections virtually freehand (without the usual cartoons).

Gianluigi Colalucci, the Vatican's chief restorer, today took journalists up on the scaffolding that cloaks the "Last Judgment" to show the sort of dirt that now masks and distorts its glorious figures.

"When it is clean, their expressions will come alive," he said. "We do not work with the idea of touching up colors or expressions. We work to remove the layers of dirt so that they can once again be seen."

Most outside experts seem to agree.

"I have no doubts about the integrity and quality of their work ... and their results," said David Bull, chairman of paintings conservation at Washington's National Gallery and a frequent visitor to the Sistine scaffolding, in a recent telephone interview.

Bull dismisses as "blatantly untrue" and "ridiculous" allegations that Vatican restorers stripped off last-minute shadings or a finishing layer done by Michelangelo.

He cites "irrefutable evidence" that the famous layer of brownish glue, originally aimed at restoring the fresco's natural luster, was put on long after Michelangelo's death. "Nor," he adds, "has there ever been a question of their removing Michelangelo's 'finishing touches.' "

Restorers, in fact, discovered a fine layer of dirt and dust -- probably decades' worth -- between the fresco and the first touches of glue, proof the extra coats came later. In addition, period art historians described the artist's colors as vivid, noting he was painting for an audience that would see his work from 60 feet below and in dull light.

Vatican restorers have added an extra arrow to their quiver to ward off future attacks on the color front: Hung prominently at the exhibit is a 16th-century copy of the "Last Judgment" by a friend, artist Marcello Venusti. Painted in oil, its bright hues have survived the centuries to act as Vatican witness today.

Venusti's painting also serves as an intriguing glimpse of Michelangelo's work before many of its nude figures were covered as part of a campaign against "indecency" ordered by Pope Pius IV in 1564, according to Vatican documents. Artist Daniele da Volterra, since nicknamed "Il Braghettone" (the drawers maker), was the first chosen to do the job.

Discreetly, he swathed the loins of many male figures, while draping the breasts and thighs of Saint Catherine of Alexandria in olive green.

Crouched behind her prone figure, brandishing the torture tools of his martyrdom, is Saint Biagio. Because Biagio's pose was deemed overtly suggestive, Volterra was ordered to turn the male saint's face away from Catherine and direct his gaze more respectfully toward the imposing central figure of Christ.

Restorers, however, insist removing the lingerie isn't a matter of morals. Despite divergent opinions expressed during the seminar of experts, they have decided that Volterra's additions are "historical documents" regarding an important period in the church -- the Counter-Reformation -- that shouldn't be erased.

Technically, the "Last Judgment" will be harder to clean than the ceiling, because it has many more layers of dirt and touch-ups; its wall position made it more accessible to restorers and their various potions, as well as closer to smoking altar lamps. Restorers will confront a new and semiprecious substance -- lapis lazuli -- that Michelangelo used for blues on the wall. In addition, its unified surface means it will have to be treated as a huge painting, rather than panel by panel as the ceiling was.

Because protective coverings eventually turn dull and damaging, restorers have opted to leave the fresco "naked" to the elements that include pollution and the breath of visitors who on a single day last summer totaled 19,000. Open windows, the only existing method of climate control, allow in soot, damp, bugs and birds.

Vatican officials said today that studies show there's "no immediate danger" to the restoration, which is holding up well, and that no limit on visitors is envisioned. Their plan to design a "micro-climate" to control temperature and humidity and filter out pollution is an idea in the works for years that has yet to materialize.

Colalucci, however, seems serene about the work ahead of him. He said the harsh attacks on his work had produced "moments of bitterness" that had taken their toll.

"Now I feel calm for the first time in many years. Like anything you work with day in and day out for 9 1/2 years, I feel intimate with Michelangelo's work. I know it to the core," he says, adding:

"I must confess I harbor a lingering, almost unconscious fear that someday, someone will come out, unexpectedly, with a really intelligent observation that will show all of us to have been blind."

Deep down though, "presumptuous as it may sound," Colalucci feels Michelangelo would be pleased.

"He'd also be happy to know that 500 years later people are discussing him concretely, not letting him languish as a legend," he adds. "It gives him new dignity ... after all, a man who can inspire fear and uncertainty five centuries later -- is more than an artist."