Retarder as the name suggests, retards or slows the drying time of the paint.
There are two ways to use this medium. The first is to use up to 3% Retarder to colour; this will result in a slightly more fluid colour that will stay wet for much longer. The second method is to put the Retarder in a hand pumped spray bottle (atomiser) and spray it onto the painting you want to keep wet. (The spray bottle should create a mist not a hard squirt).
Additions of drying retarder to mixing water may be used in water colouring, water wash effects, stain painting and hard edge techniques where it assists in pigment wetting, rapid colour diffusion and penetration.