Plexi makes a wonderful surface to paint on. You can use both sides and take advantage of translucent layers of paint, play with opacity and transparency and layer colors in a unique way. Be sure you varnish after you're finished to protect the surface from possible scratching. Golden makes a special purpose polymer GAC 200 that is useful for working on non-absorbent surfaces like plexi. Use it as a clear gesso that will increase adhesion of the acrylic to the plexi surface. Then away you go! Paint to your heart's content, knowing that the paint will adhere to the surface.