Understanding oil mediums: Thickened Linseed Oil and Stand Oil. Both are great for creating reliable mediums for working in layers. Thickened Linseed Oil is polymerized in the presence of oxygen to make it fast-drying and flexible; it is an alternative to sun-thickened linseed oil which is difficult to source today. Stand Oil is heat polymerized in a vacuum, making it flexible, non-yellowing but slower drying. Whilst both can be used to work from thin to fatter layers, Stand Oil is slower drying and therefore is best used in the final layers of the painting.