Atelier Interactive Acrylic Additives Atelier Interactive has added even more creative freedom with their line of mediums. Acrylic Mediums are used with acrylic paint to manipulate the consistency and properties of paint for an infinite variety of applications and techniques. Acrylic Paint Additives, Like mediums also change the properties of paint, however they are different than mediums in that they do not contain a binding agent. This means that only a certain amount of additive can be mixed with acrylic paint before it becomes unstable. Choose from Atelier Slow Medium, Clear Painting Medium, Fast Medium or Fixer, Atelier Unlocking Formula and Retarder.