Melissa McGrew balanced herself high above the floor of Old North Church, perched at the edge of slightly rocking scaffolding as she burrowed through paper-thin layers of paint and plaster to find the church’s original skin.

A good day is uncovering 7 square inches.

It’s nerve-racking work for the conservator, a baby step toward an estimated $8 million renovation that will bring the building up to code. Along the way, the interior will get a radical cosmetic makeover in time for the church’s tricentennial in 2023 and, two years later, the 250th anniversary of Paul Revere’s famous ride.

“It’s very stressful,’’ McGrew said, dabbing and wiping and cleaning with a painstaking touch. “But I love it.’’

An 18th-century object of her affection lay before her, a winged cherub painted shortly after Old North was built in 1723. Because of McGrew’s tortoise-pace work, its rosebud smile and double chin have reemerged after being hidden for more than a century.

The cherub is just one original detail that will be visible to the public again, a byproduct of the overhaul that will “rip the walls apart,’’ said the Reverend Stephen Ayres, the Episcopal vicar at Old North.

The project will be a mess, Ayres said, but one that gives church officials a rare opportunity to study what lies beneath the walls. The cherub, for example, is possibly one of 20 such figures on the nave’s upper arches.

The work has made detectives of McGrew and fellow conservator Brian Powell, who — incision by incision, and swab by paint-dissolving swab — are collecting clues about how the church has changed over nearly three centuries. The pair, who have been at work for about three weeks, plan to spend two months at the church.

So far, Powell said, their archeology shows that the walls once were bathed in a reddish wash. That hue would have served at least two purposes: to add color to the interior, and also to create an illusion of hardwood.

Later in the church’s history, oversize paintings of sumptuous, parted drapes filled much of the space above the organ. Such decoration often was found in the Church of England, which included Old North — officially named Christ Church — at its inception.

“That’s the great irony: One of the king’s churches betrayed the king,’’ Ayres said.

Today, the florid finishing is long gone, hidden under a blanket of white applied during the last big renovation of the church in 1912. The century-old coating “really doesn’t bring out the richness of the architecture,’’ Ayres said.

“They overshot in the other direction,’’ said Powell, his eyes rolling slightly at the change. “They said, ‘Hey, let’s make a Quaker meetinghouse.’ ’’

Ayres said that findings by Powell and McGrew, who work for Building Conservation Associates, will help decide which period the church will reflect. Perhaps Old North will return to its appearance in 1775, Ayres said. Perhaps the look will come from another moment in time.

Whatever the change, the effect should be startling.

“These columns are going to pop’’ with color, Ayres said with a smile.

In the meantime, the conservators will continue poking and prodding, often using nothing more than delicate pinpricks to gauge what has clothed the church’s skeleton. Powell, a 20-power magnifying device draped around his neck, estimated that at least five layers of paint and plaster coat the walls.

“This is fabulous, for us to have a time capsule like this,’’ Powell said. “We’re here to expose these wonderful things.’’

Obscured by the Revere legend is the architectural importance of Old North, which Powell called “the first important Georgian-style church of its type in English North America.’’ That design was inspired by the great English architect Christopher Wren, whose St. James’s Church in the Piccadilly section of London served as a model for Old North.

Its lines still inspire. McGrew said she attended a recent Sunday service at the Boston church to immerse herself in its aura and atmosphere.

“I learn something new every day,’’ she said. “It’s thrilling.’’

She also uses that word to describe her work, although the thrills can mix with eye-taxing tedium. But as she works, tiny space by tiny space, her passion is as evident as her attention to detail.

“This is a skill that was handed down through generations,’’ McGrew said of the cherub, its eyes tilted heavenward. “It’s almost a lost art.’’

Brian MacQuarrie can be reached at brian.macquarrie@globe.com.