An intensely detailed process, it brings together past and present in this huge light-filled studio at the end of a bumpy road in a nondescript Glasgow industrial estate.
It is a rather incongruous setting for works by Bellini, Titian, Rosa and Botticelli but they have all passed through the studio from the stores of the City of Glasgow's art collection to exhibition displays at Kelvingrove Art Gallery and Museum and around the world.
"Our remit is much bigger than the hands-on treatment of paintings because we work across all of the storage and museum venues in Glasgow," explains senior conservator for the art group, Polly Smith. "It is to preserve the collection, maintain and improve the condition of the work, so that we are making it accessible for future generations.
"We define our work as trying as far as possible to show the paintings in the way the artist intended them to be seen but with respect for the fact they may be 300, 400 or even 500 years old.
"We want them to look as good as something could be if it had been well looked after for 500 years rather than looking as though it's brand new and painted yesterday. It's all about respect for age and change of the material."
Between Polly and her colleague Suzanne Ross at their base in the Glasgow Museums Resource Centre they keep a careful eye on 4000 easel paintings ranging from Italian 14th century to the present day. Just now they are working on a group of paintings for a re-display at Kelvingrove and for a large temporary exhibition next year to replace the current Vettriano retrospective.
"We worked on every one of the paintings in the Essence of Beauty: 500 years of Italian Art exhibition to varying degrees," says Polly. "One of the most dramatic was The Adoration of the Magi, it hadn't been on display for at least 25 years."
Suzanne adds: "Discoloured varnish was removed to show the original colour of the paint. It completely changed the colour balance within the composition."
As technology has changed over the years so have the techniques of an art conservator. Much of the work carried out by Polly and Suzanne is examining paintings to better understand their condition and the materials the artist has used. Over 20 years with Glasgow Museums, Polly says her work has altered dramatically - in location and approach. From a small studio at the top of a spiral staircase in Kelvingrove to today's purpose-built space in Nitshill, the modern minimal approach acknowledges the fact that every time she intervenes there is potential to be changing a picture irreversibly.
"We try to do nothing that cannot be reversed," she says. "If we have an area of a picture that needs work we are particular to restrict our input and retouching to that small area of loss. In the past there was a much freer sense of retouching."
Currently on display in Kelvingrove, Norah Neilson Gray's Portrait of a Girl in Blue shows the magic of a conservator's work - a dirt layer on top has been removed to reveal a discoloured varnish layer and finally the true colours.
"However many years you've been doing the job, that's why it's still exciting," says Polly, who explains that x-rays and infra-red examinations often reveal under-drawings, re-paintings and composition changes, made by the artist.
Work on a painting can take anything from a few days to several years.
The conservators were involved in the current Vettriano exhibition, even though most of the paintings were relatively new. Their duty of care means the condition of every painting has to be logged from arrival to return.
Councillor Archie Graham, chairman of Glasgow Life, said: "Only a small proportion of the 1.4 million objects in our collection can be put on display at any point, but the public are always welcome to explore our hidden treasures at the Glasgow Museums Resource Centre.
"It is a treasure chest full of objects covering art, natural history, transport and technology, which you can discover by joining one of the tours and activities."