RE: using expanding foam in built up wings
I would go to the nearest model flying field at the end of the day on a weekend where there are a lot of flyers and rummage in the trash cans for trashed/crashed balsa airplanes. Take all you can as you may need to do a lot of experimentation. Pick up the wing parts and take it home and spray the expanding foam inside the wing as an experiment and see if the expansion distorts/damages/ bulges the wing at all. If there is distortion/damage/bulging, then try drilling escape holes on the underside of the wing surface and see at your next experiment if the escape/relief holes will allow the expanding foam to escape and stop the distortion/bulging of the wing. If it does then you have the solution. It is always a good idea to pick up crashed balsa airplanes for your experimental repair jobs. Nothing beats trying it on a real balsa airplane.........even if it is a crashed version. 'Nuff said.
God Bless y'all, always
larry