So I had more than a bit of a scare today: got in a painting of an archangel in the Cuzco style here, and it arrived in grievous condition:
It was covered in some sort of scuffing or scraping:


But it turned out that it was actually just dirt that could be carefully wiped off using some makeup pads and distilled water:
And in fact the entire painting seems to have been covered in more of that same grime, just not as obviously. I wonder if it was cigarette smoke from a century in France or what. After cleaning it fairly thoroughly, the final result turned out quite well:
I was lucky though - nobody seems to know
anything about how to clean unvarnished oil paintings, it's effectively luck of the draw. Older paintings such as this one (19th century) are fairly indestructible, but modern oils seem to be capricious and poorly understood. It's an active area of research just to figure out whether water or mineral oil is the best thing to use on contemporary paintings.
If it happens to be varnished, on the other hand, you can scrub it down with any number of commercially available salves, like so:
The moral: varnish your paintings, I guess. It was never an option on this particular piece though, due to the
impasto elements.