Thinner. Turpentine oil or rectified turpentine is a proven product for diluting oil paints and cleaning brushes. It can also be used to thin varnishes and thinners for oil painting. It evaporates without residue and does not form a film. Slightly accelerates drying. Turpentine is quite strong, so there is no need to overdo the amount used. Beware of its volatile properties and smell, it can be unpleasant for allergy sufferers.