Newbie painter here. I have been given a lot of paint cans, but most of the labels are obscured. I am trying to figure out if each can is oil-based or latex (for the purposes of cleaning the paintbrushes afterwards, as each type requires a different cleaning solution). I am told that the rancid smelling cans are the oil-based paints. Is this a hard-and-fast rule?
Normally, latex paint is water soluble while oil paint isn't, so trying to dissolve a drop of paint in water should give you an indication. Don't put water in the paint bucket; if it turns out to be oil base you'll ruin it.