Solvents are used to thin oil paints before applying them to the surface, turpentine and turpentine substitutes are essential items in the oil painting workspace. Oil medium is used to thin the paint before applying it to the canvas. A traditional medium is a 50/50 solution of turpentine and linseed oil. There are many variants to this mixture and many commercially made mediums are also available. Adding a medium to your paints not only thins them, it also gives your work a glossy reflective quality when dry.