CafeenMan is right on. I've been building powered model aircraft since the 40's and I always prime and paint the balsa surfaces using aircraft dope. Strengthening is unnecessary and I don't believe iron-on film does much of that anyway. Fiberglass adds strength but also a lot of weight, which may be appropriate for very large models but usually not. My current procedure is a prime coat of laquer based sanding sealer, two thinned coats of clear dope, sanding lightly between coats and two sprayed coats of color dope. The resulting coating is lighter than film and looks good. If butyrate dope is used, it is fuel proof. There are also no seams to lift.