For internal hardening, I use wood glue. For the exterior I am going to try glue a thin layer of chipboard (aka Eggo boxes 0.45 MM) to the existing hull exterior. My hope is that the layers will be strong enough. If not, I may apply a layer of mod podge and seal it with primer and paint. As I get closer to that I'll do some test samples to see if that will work or if I need to initiate an alternative path.