Non-invasive cleaning
 

If you're not looking to actually strip layers from the surface of a painting, but just clean up an old painting that has collected dust or soot or bad karma, or prepare a recently-dried painting for a final coat of varnish, I recommend a 3M product, Scotch-Brite(TM) High Performance Cloth. It's chemical free, virtually lint-free, and cleans by operation of scratch-free microfiber technology. Because it's the structure of the fiber that cleans, and not any added solvent or cleaner, it can be repeatedly laundered and continues to perform. Many of you are probably already using it for its "intended" purposes, which include lens and computer screen clean-up, as well as polishing myriad household and commercial surfaces. If you haven't seen or tried it, just look: it's everywhere-- department, drug, DIY stores and elsewhere. Some info can be seen at www.3M.com/highperformancecloth. In the interest of full disclosure, yes, I do happen to own some 3M stock. But of course that's not my point.

Non-invasive cleaning
 

Steve,

Thanks for the advice. I looked all over and finally found cloth similar to that you described in a drawer.

I have an oil painting that I bought at auction, very old and dirty. I used the cloth dry and was able to pick up dirt. However, I tried it wet with a little detergent (which I rinsed out after applying it) and that made the difference. Using the cloth wet really made a difference in picking up the dirt off the surface. The painting has really freshened up. Thanks.

Gloria