Paper Hardener

they say...

found this stuff: says "Paverpol [fabric hardener] can be used to make all sorts of materials rock-hard, such as textiles, paper,..."Also, I think any clear-drying decoupage glue (like Mod Podge) would also harden paper.

*Reminder: Paverpol_ Gotta check this out on my next trip to Artfriend

An excellent fabric hardener that dries fast and is very easy to use on any natural material such as silk, soft leather, self-hardening clay, polystyrene foam, wood, plaster, ceramic and more. It adheres to almost anything except plastic.

Unique Qualities:
Dries rock hard without additives.
Causes item to be weatherproof and suitable for outdoors.
Washes off with warm water.
Mixed product does not harden; if kept in sealed container it can be re-used.