Art Spectrum Oil Painting Medium 3 is the fattest medium. It is a thicker, slower drying medium than Medium 2. It is for painters who like a heavier, more viscous medium. It is ideal for artists who paint in many layers. The artist begins with Medium 1 than progresses to Medium 2 and then finally Art Spectrum Medium 3. Medium 2 may be gradually fattened by adding increasing amount of Medium 3 in 10% increments.