Use Dick Blick's Matte Acrylic Varnishes over Blick Matte Acrylic Paint, or any other craft acrylic paints. They provide a tough, non-yellowing protective coat.
Matte Varnish — This varnish provides a protective finish coat of toughness and flexibility. It is clear, non-yellowing, and strong enough to resist chemicals, heat, and abrasion. Cleans easily with water. 2–6 coats recommended. Plastic bottle.