Another direction you could go is with a two part product called Styro Spray 1000. It is most commonly used in making styrofoam props for movies. There is an outfit that sells it in small quantities under the product name Liquid Skin at wowplanes.com. They sell it specifically as a covering for foam. It creates a plastic like smooth surface on foam. Tfarmer, here on RCU has been experimenting with it for use with balsa instead of traditional fiberglass and resin. In this application, he thins the product before applying to foam. It can be thinned with denatured alcohol. His thread his here: http://www.rcuniverse.com/forum/m_10131013/tm.htm
If you do thin it, be sure you are using denatured alcohol, as the product is set off with moisture, and isopropal, or rubbing alcohol have water which will ruin it. Styro Spray is sold in a larger quantity, but is cheaper per ounce. I would still suggest the small quantity packaging from wowplanes, even though the cost per ounce is much higher, once you open the containers, it has a fairly short shelf life, and may save you money over buying the smallest packaging of Styro Spray.