I recently finished up casting over 20 dozen foam decoy heads from a 5 gallon kit of 12 lb density two part foam. The heads are a mix of puddlers, divers, and some goose heads. After some research I decided to use a restle coating on the heads.
The coating is simply placing a glue on the head and then spreading a coating of fine wood dust or fine ground corn cob over the glue. You hang the heads to dry. Once they are dry you dilute the glue with water at a ratio of 60 percent to 40 percent and brush on another coating of glue.
The Restle Coat leaves a very hard, tough textured finish on the heads.