I cover my foam wings with silkspan and thinned carpenter's glue prior to the iron on film. It doesn't add much weight, it stiffens the wings considerably, improves the surface appearance, and improves the covering's adhesion. If you've never done it, here's the quick overview. This assumes you are starting with a plane that would otherwise be ready to apply the iron on covering.

1)Thin some Titebond glue or similar with water, approx 1 part glue to 1 1/2 parts water. The exact ratio doesn't seem critical. Put it in a wide mouth jar, or a cereal bowl to make it easier to use.
2)Cut 4 pieces of medium silkspan to size. I cut them to the same size as the foam shucks, but about 1/2" more chord. Cutting them to size first is much easier and faster in my opinion, but you can cut them extra big and trim them later if that's how you like to do it.
3) lay a piece of silkspan on a towel and spray it with a water bottle until it is uniformly wet, but not dripping. Blot it on the towel as required.
4)Position the damp silkspan on the wing panel so that there is about equal overhang on the leading and trailing edge. Using a 2" foam brush, paint the thinned glue onto the silkspan. It WILL soak through to the foam. Scrape the surface with a playing card, working  towards the TE and wingtip to remove the wrinkles and excess glue. Wrap the overhang around the TE.
5)Immediately do the other side of the same wing panel in a similar manner. This will avoid curling the trailing edge due to uneven shrinkage. Once done, the covering can be blotted with paper towels if it is still excessively wet.
6)Repeat for the other wing.
7)Let dry for a day, and sand lightly with 220 paper. If you sand through anywhere, smear it with a bit of un-thinned glue. Marvel at how much smoother and stiffer the wing is.
8)Wipe the wing with a tack rag, and proceed to the iron-on covering.

I have not done this on a stunt airplane, only carrier and combat, typically 300-450 sq. in. The wings are typically solid foam, occasionally cut out for internal controls. I have not measured before/after weight. You can weigh the silkspan, which is next to nothing, so really the weight is all in the glue. If you start with 2-3 oz of glue, some of which is water which will evaporate, add more water which will evaporate, and then consider that the majority of the glue gets scraped off anyway, there's not much weight there.