So, you want to try to clean an old oil painting. There are two main reasons why older paintings might look yellowish, dull, or dirty. First, they might actually be dirty! Secondly, older oil works were typically treated with a varnish coat to add sheen and gloss. This layer of varnish does protect the paint, but will darken and lose clarity over the years.
If want to risk removing the dingy varnish, most specialty art stores sell an emulsion designed to do just that. You should first try dabbing just a tiny bit of the solution in a discreet spot as a test. If you suspect that you can successfully remove only the varnish without harming the paint, proceed to work on larger swatches. Remember, there is considerable risk of damaging the painting or reducing the value and purity of the antique.
To remove common dust and dirt build up, a clean, very damp (not wet) cloth will do wonders. Some people have suggested white bread, or white bread dough to use as an “eraser” over the surface area. For everyday dust and hair, a vacuum cleaner with a very soft-bristled attachment set on a low suction setting is a perfect method. Search iantiqueonline.com for other suggestions from the antiquing community.
Good luck, and happy collecting!