Amsterdam Acrylic Matt Medium by Royal Talens is the ideal medium to use if acrylics need to be thinned out without loosing the durability or drying time of the paints. The Acrylic Matt Medium dries colourless and transparent and can increase the flexibility of the paint after drying.
Matt Medium can be thinned with water, but becomes waterproof when dry, and reduces the gloss from Acrylics to leave a more Satin-style finish.