Matte Medium is an Acrylic Polymer for use with Acrylic paints. If you use it individually, you can get a clear finish. The other way to use it is to mix it in varying amounts with Acrylic paint, which will change the natural gloss result of Acrylics to a more matte finish. The brand I use is called Speedball, which is made by Hunt, and usually available at artist's supply stores like Dick Blick. Liquitex is another brand that sells a similar product.
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Gordon Moat Photography