RE: 42"wingspan p51d what size engine?
Paper & glue works over foam to add rigidity like a poor man's fiberglass, but since it doesn't shrink, You'll get sags between ribs and have a mess to remove. And since it won't shrink, you'll lose the compresion loading and attendant airframe strength film gives. This will seriously compromise the strength of the airframe. Don't cheap out now, Lighter flies better, and the best bang for the buck will be iron-on film. I really like easy-coat low temp stuff, and it's made in Germany, so maybe it's easier to get there than here. BTW, if that .10 causes nose heavy or clearance problems, get a Norvel .074...Super light & compact with more power than a .10.