I am building a model rigid airship, and in order to maintain it's structure I used a balsa frame. Unfortunately this weighs a fair bit and can take up to half of the lift generated from the light gas, and it is not particularly strong. Since I use polyurethane for the outer skin, I am wondering whether it would be possible to inflate the airship to the size I would like it to be, and then treat this material with some kind of chemical to make it hold it's rigidity? I would be open to any kind of method that is similar to this and ends up giving me a lightweight rigid structure. I also know that this may be not possible, but any ideas I would love to hear about.