When restoring an oil painting, I have a responsibility to the person who painted the piece, so that his effort is restored and preserved. I, also, have a responsibility to the person who will own the painting hundreds of years from now. Of course I responsibility to the current owner of the painting, effectively I'm working for three people. First, Do no harm, Second, Do nothing that cannot be undone.
I inspect the painting looking for obvious problems, holes, missing paint, dry rotted substrate, is it just dirt? Oil paintings come on canvass, linen, wood, Formica, canvass board, masonite all of which have their own benefits and problems to be aware of. Depending on what problems I find, I advise the customer what should be done and what the cost would be.
Starting the process, having determined that the canvass is in good shape or having repaired the canvass, using a cotton ball to apply a light coat of restorative oil on the surface of the painting softening the dirt and original varnish and providing lubrication for the actual cleaning process. Picking an area approximately the size of your palm in the background near one of the edges and using a cotton ball to carefully apply a small amount of cleaner and with another cotton ball to wipe off the loosened dirt. Once satisfied with that palm sized area move to an adjoining palm sized area until the entire painting is cleaned.
Some areas will require more care, some pigments are more delicate then others, the artist may have not varnished that area. These areas will require many light cleanings allowing the paint to harden in between cleaning. The idea is to remove the dirt without removing the paint. Once the painting is cleaned, it left to dry, the drying period depends on the relative humidity, until all the solvents have evaporated and the paint surface has hardened. Then a varying number, dependant on how much of the retouch varnish is absorbed, of coats of retouch varnish are applied until a uniform sheen covers the entire painting. If the painting was just dirty, the painting is now ready to go home for three to six months, to completely dry. Then it will be returned for a day to be dusted off and a coat of damar varnish is applied and the job is finished.
The cleaning of the painting is exacting and tedious, but not particularly creative, unless somebody used a non standard paint, over the original paint or glazing. Then it becomes a mystery to be solved and dealt with.
The creative aspect of restoration is the repairing of the canvas, linen, what ever substrate the artist chose to work on. I personally enjoy the reweaving of the old fibers or searching for a canvas, linen that matches the old canvas, linen to replace the missing material. And bringing the new surface up to the same thickness as the original so that it is as seamless as possible. Some of the paintings I've worked on came to me in pieces, the canvas was so brittle it was like a potato chip, the canvas shattered when the framed painting was dropped. Putting the painting together was like working a jigsaw puzzle, except the edges of each piece had to be woven back to the adjoining pieces. Then the cleaning can begin, the same as above.
Once cleaned, the painting is revarnished until the painting has a uniform sheen, then allowed to dry. The coat of retouch varnish separates the old work from anything I do, so that, for what ever reason, any retouching that I do can be removed without interfering with the original painting. At this point in the restoration the fun begins, figuring out what colors, mixtures of colors and tones the artist used to achieve the colors. The wet paints may match the original paints while wet, but not when dry. And the new paint may not match the old after varnishing. The technique used by the artist may use a thicker or thinner layer of paint, that has to be allowed for. Some of the older pigments are different then today, do to impurities or what ever the pigment was the artist was grinding previously that was left in his mortar and pestle. Some of the older pigments aged not so gracefully and the pollutants have altered the original colors. Each painting presents it's own quirks, just like people.