A viral video showing hundreds of years of varnish being wiped off a centuries-old painting has horrified experts in the field.

The short video was posted on Twitter by the art dealer Philip Mould, who hosts the BBC TV show "Fake or Fortune?" and was involved in the cleaning process, according to statements he's made to other news organizations. The clip shows someone glopping a gel-like substance directly on the face of a Jacobean lady pictured in a 1618 portrait and scrubbing it away with what looks like a paintbrush, almost immediately revealing the vibrant colors of the mysterious face beneath the aged yellow surface.

But the short video has shocked and horrified art conservators, who say the video gives a false impression of the painstaking and methodical methods normally used to clean old paintings. What's more, using the technique as depicted in the video could strip away the artwork's underlying paint, permanently damaging the painting, two experts in art conservation told Live Science. [11 Hidden Secrets in Famous Works of Art]

"The thing that was so terribly disturbing about that video is the guy starts right on the face of the sitter. When I saw this video the first time, after about 10 seconds, I just had to turn it off because I couldn't watch it anymore." said Rob Proctor, a conservator at Whitten & Proctor Fine Art Conservation in Houston.

The sound in the video was particularly disturbing, Proctor said.

"You can just hear him scrubbing," Proctor, who has run workshops on varnishes, told Live Science. "It's a beautiful, old, culturally important painting. And then to watch the stuff just drip down the surface of the painting? It's really crazy."

Painstaking and methodical

Historically, almost all old paintings were coated with varnishes such as mastic or dammar. These materials, made from tree resin, essentially "wet" the surface of the painting and give colors more saturation, depth and definition, Proctor told Live Science. But over time, those varnishes age, leaving an ugly, dull-yellow film. The centuries also leave their mark in the form of dirt and grime on a painting's surface. [In Photos: Van Gogh Masterpiece Reveals True Colors]

When conservators clean paintings, the goal is to remove the yellow varnish and dirt, returning the image to a state as close to the original painter's intent as possible. Usually, they examine the surface under a microscope, and use research or background knowledge of painting materials and techniques to select safe, effective varnish-removal materials, such as a gel or a solvent, Proctor said.

Next, they test that materials on small patches on the edge of the painting, before slowly proceeding inward toward the heart of the artwork, he said. Test patches allow the conservator to determine whether the solvent can simply sit on the painting to remove the varnish, or if mechanical action, such as rolling a cotton swab, is needed to remove the varnish, Proctor said.

What's more, there may be multiple layers of varnish or coating on a given painting, and the same solvent or gel may not work for the whole painting, necessitating removing the surface layer-by-layer, Proctor said.

Overcleaning and damage

If conservators aren't careful, they can ruin ancient works of art by overcleaning or damaging the underlying paint, said Gwen Spicer, of Spicer Art Conservation, LLC, in Albany, New York. For instance, past attempts at conservation heavily damaged the Leonardo da Vinci painting "Salvator Mundi," which just fetched $450 million at auction this week, with particular overcleaning damage in the face of Christ, ArtNews previously reported.

In the case of the new video, one of the upsetting elements is seeing the gel dripping down the painting in an "uncontrolled flow," Spicer said. That's because different parts of the underlying paint may absorb more of the solvent or gel and be more swollen than other parts, so letting the substance just drip down the painting runs the risk of affecting the paint layer in one place even if other parts of the painting are fine, Spicer added.

Also, the surfaces of paintings aren't perfectly smooth, but have raised or recessed areas. So, using the mechanical action depicted in the video could be fine for some areas of a painting, but could damage the surface in others, Spicer told Live Science.

In addition, gels to remove varnish may leave residues that, if not cleaned off, will mix with the underlying paint and change its chemical composition. Left on the painting, these residues might not affect the painting initially, but could cause damage 50 or 100 years later, Proctor said. The short video doesn't show the techniques used to remove the gel.

The mechanical rubbing action shown in the video isn't how modern conservators are taught to remove material, Spicer said. Typically, people will roll a cotton swab to lift up the layers of varnish, rather than scrubbing the painting, she said.

Cavalier approach

The new video depicts the conservation process as hasty, casual and dramatic — the opposite of how most reputable conservation projects proceed, Proctor said.

"We're reacting to this cavalierness," Spicer said.

Mould probably did some work behind the scenes, or had worked on so many similar paintings in the past that he had a good idea of what type of solvent or gel would be needed before he got started, Spicer said. For instance, in the video, the varnish looks very thick, so it's possible Mould had previously looked at the painting's cross section and found there was a protective layer underneath the varnish layer, which isn't explained or shown in the video, Spicer said.

"Probably, when he tested, he looked at the cross section and could see that there was possibly another layer," Spicer told Live Science. (In fact, after the video caused an uproar, Mould clarified in statements to the Telegraph and on his Twitter feed that he had done extensive testing on an oak panel and carefully selected the solvent and gel prior to making the segment.)

However, even giving Mould the benefit of the doubt, the way the cleaning appears to have been carried out could potentially damage the painting, Proctor said. And the video gives a false impression of what goes into the process of art conservation, Spicer said.

"It's an unfortunate thing that it's gone viral when there are a lot of other things that would be far more representative," Spicer said.

Originally published on Live Science.