There is a product made especially for this. The following is copied from the WASCO website at
http://www.taxidermy.com/cat/18/artwater.htmlCastin' Craft Resin Surface Coat SprayIf you do any habitat work with Artificial Water or other resins, you will really appreciate this multipurpose product. As Frank Newmyer said, "This is really great stuff!" Simply spray this over any resin casting, water surface or splash to instantly cover any imperfections -- cracks, fingerprints, dull areas or attachment points. Gives a smooth, glossy, tack-free finish to any resin surface in one easy step. Dries absolutely clear with a hard protective gloss. Fast drying. You can even use it to quickly coat decorative items like sea shells. Great for applying as a final coat on acrylic bird eyes to bring out the shiny wet look.