It took 20 years to do it, but the Sistine Chapel has been cleaned, polished and studied down to the last corner. Now, it's ready to welcome the throngs expected to visit during the Holy Year Jubilee in 2000.

On Saturday, Pope John Paul II will bless the chapel that has been at least partially covered with scaffolding for most of the past two decades while experts washed away the dust and grime accumulated over five centuries of Vatican life. All of the masterpieces' colors shine bright, and restorers say the work should hold up for decades.

"Every inch of this chapel has been restored. . . . and it's overwhelmingly beautiful," Cardinal Edmund Casimir Szoka, who oversaw the restoration, said during a preview.

Restorers didn't know what they would find once the work began. But they uncovered a Michelangelo that was more colorful than imagined under the dark and nearly murky figures of the "Last Judgment" on the side wall and beneath scenes from Genesis on the ceiling.

The latest phase of the project--the $3 million cost was paid largely with private donations--was devoted to the cleaning of the side walls. They show a new grace and vitality in the series of 12 frescoes depicting the lives of Moses and Jesus. The illustrations were created by 15th-century masters preceding Michelangelo, most notably Botticelli, Perugino, Ghirlandaio and Cosimo Roselli.

During the clean-up, restorers learned to recognize the styles of the artists who apparently worked side by side, with dozens of helpers, to complete the pieces commissioned by Pope Sixtus IV in 1481-82. By tracing the black holes that indicate the "cartoons" or drawings that were sketched onto the wet fresco matrix for the painter to follow, "we now know that Perugino economized on paper . . . he made cartoons only of the figures in the forefront, he painted freehand on the landscape," said Arnold Nesselrath, who heads the Vatican's art department.

Botticelli used red outlines to draw the three frescoes attributed to him, including the "Temptation of Christ" and "Trials of Moses," Nesselrath said, while Ghirlandaio played games with his name, giving some of his figures ghirlandi, or garlands, on their heads.

"We see now how the color scheme worked," he explained. "The blues in the landscapes of these paintings are the same blue that Michelangelo used in his 'Last Judgment.' "