Steve if real light is the goal go for Real Silk . the cost is bigger and not as easy as Silkspan to work with but the weight saving is even better.
Silk is the strongest lightest covering you can use and yes two to three seal coats and one color coat will work and still be less weight than Iron on film.
Sig sells Silk fabric for planes just look around or order direct from Sig.


Good luck


Den


Oh you can look here too http://www.modelflight.com/weight.html