Retarder medium is a slow-drying medium that slows the drying time of acrylic paint. Only a tiny amount is needed, up to 5%. It does not alter the colour or finish of the paint. The length of the extended drying time is affected by temperature and humidity.

Additions of MM1 Drying Retarder may be used in watercolouring, water wash effects, stain painting and hard edge techniques where it assists in pigment wetting, rapid colour diffusion and penetration. On raw canvas, the addition of 3-5% MM1 Drying Retarder, pre-diluted with water, will improve the mixing of inorganic pigments, oxides, earth colours, titanium dioxide and metallic pigments.

MM1 Drying Retarder will slow the evaporation of water from the paint. However, excess amounts of MM1 Drying Retarder (over 10%) will severely slow "cross linking" of the acrylic. Therefore, even though the paint may look dry (and indeed all the water will have gone from the paint), it will not have "cross linked" or "cured" so it is able to be re-wet, i.e. with a wet brush or cloth. This allows the Matisse acrylic colours to be used just like watercolours.