I have a no-food-on-the-loft rule and a huge oil painting hanging beside the stairs. Looking closely on it, I saw a lot of stains on it - probably the Muscat wine the policeman smuggled up there. Rules don't bind men of the law ...I tried with a soft detergent first, but to no avail, the remembered a trick my grandmother had used on a very old painting: cut an onion in half and carefully rub it over the painting, cut off a slice from time to time. It takes everything away that was added after the painting was varnished of finished to fix the oil colors. Stains, patina, dirt. I wiped it with a damp soft cleaning towel and it looks like new.