Here's the answer... and it IS a great question. Oil and acrylic painting have many painting techniques through the ages that make them susceptible to damage if you don't know what you are doing.
Added to that is what ever is on the surface (nicotine, discolored varnish, grime... varnished over grime, lacquer, linseed oil varnish to name a few). Each is taken off in a different way.
Those two conditions (technique plus the type of stuff you want cleaned off) make cleaning pictures the main reason and way that paintings are often damaged by inept people.
The only cleaning technique I can offer, then, is saliva and Q-tips! You might say, "Ewwww! Spit on my painting!?!?!?! Well, the enzymes will help remove grime and by using saliva, you won't use enough water to damage the art.
We sometimes clean paintings with saliva inch by inch and, in fact, may have a 3'x4' French Impressionist painting worth $millions coming into the lab in about 10 days that we will clean with this exact technique. The attached photo shows a painting during cleaning that took hours and hours to remove with a number of toxic solvents and lots of know how.
See the time lapse video of the cleaning on this page: http://
Art conservation questions? Call Scott M. Haskins 805 564 3438
Art appraisal questions? Call Richard Holgate at 805 895 5121
Questions about the painting in this video? Call Steve Stern at 310 729 6666
Follow us on Facebook at Fine Art Conservation, Scott M. Haskins and Tips for Art Collectors