The restoration of the 16th century masterpiece in the Vatican is aimed at bringing back to light the vibrant colours of the work, which measures nearly 14 metres (46 feet) high.

Visitors will still be able to access the Sistine Chapel during the restoration but the fresco is obscured by a large scaffolding covered with a reproduction of the work.

Vatican Museums director Barbara Jatta described the white layer being removed as "a bit like a cataract", during a press tour on Saturday of the project, due to be completed before Easter.

Jatta said the layer covered "the entire 180 square-metre surface" of the work.

The Vatican Museums said in a statement that the substance was "invisible to the naked eye" but had "dimmed" the original colours of the piece.

The fresco is being dabbed with distilled water through a layer of Japanese paper to remove the substance, identified as calcium lactate.

"Perspiration has increased in recent years because of climate change. Due to perspiration we produce lactic acid... which becomes calcium lactate," Fabio Morresi, head of scientific research at the Vatican Museums, told reporters.

Vatican Museums staff said measures had already been taken to reduce the number of visitors present at any one time in the Sistine Chapel, which is also the place where cardinals meet in closed-door conclaves to elect new popes.

- Part of overhaul -

Morresi described the difference in the fresco before and after the treatment as like "two different worlds" and said working on the masterpiece was an "emotional" process.

The Last Judgment, which was painted between 1536 and 1541, is the centrepiece of the Sistine Chapel, and is located just behind the altar.

Then-pope Paul III was said to have been so impressed by the work that he fell on his knees and ask for divine forgiveness when he first saw it.

Of the fresco's 391 figures, many are nude or semi-nude, which caused scandal at the time.

Many were covered up with cloths painted over the original following Michelangelo's death.

Some of the painted cloths were removed in 1994 during the last major restoration.

The current project is being sponsored by US donors and is part of a major overhaul of the Sistine Chapel that began in 2010.

Work is normally carried out when the Sistine Chapel is closed to visitors and without the need for scaffolding but Vatican Museums officials said this was not possible for The Last Judgment because of the scale of the work.

Morresi said the restoration felt personal for him because he was hired in 1988 when the last major facelift of the Sistine Chapel was just starting and he is now nearly at retirement age.

"It's marvellous... There's a piece of me in here," he said.