Indurate

Historically, "indurated fibre" (like in Fibrotta products from the early 20th century) was a type of wood pulp product that was hardened and waterproofed, often used for items like fire pails or washbasins. To give you the best method, what are you trying to do? Are you doing origami and need crisp folds? Are you making paper mache/models that need to be durable?

Let me know your project, and I can suggest a specific technique. indurate

Soaking paper in a mixture of powdered sugar and water, then allowing it to air-dry, makes the paper rigid, which is popular for origami. Are you making paper mache/models that need to be durable

Here are the primary methods for indurating (hardening) paper: Here are the primary methods for indurating (hardening)

For extreme hardening, particularly with papercraft or Pepakura models, coating with fiberglass resin is a common technique.

Are you trying to make paper look and feel like ?