Rough up the existing eyes w/ fine grit emery cloth. Paint w/ some Testors "gloss coat" or any other hobby coat. When dry, go over w/ the emery, paint w/ Future acrylic wax. Sounds involved; but if you get all of these things together, this would take about an hour or less per decoy. If the eyes are colored (like wood ducks), paint in a gloss black pupil before you paint on the gloss coat.