Brew,
There is always the old silk-span and white glue that works pretty well. Thin the white glue to the consistency of clear dope and apply a coat to the entire surface (use foam brush’s and toss them when finished). When dry, them apply the silk-span wet but only lightly damp. Attach the silk-span around the outside edges and trim off the excess covering. When that dries, apply to the bottom surface. When the wing is covered and dry, brush on a coat over the silk-span and allow to dry throughly and lightly sand the surface. At this point you can, if desired, and a second coat of the thinner whit glue. Not the fastest way but it does work well on foam, then you can use a spray on rattle can finish. In the past I’ve used this on free flight models and they held up very well. You could also try the C/M and Tight-bond II method that Bobby Hunt uses also.
Later,
Mikey