I make sculpture from handmade paper and I'm wondering if there is something I could add to the pulp or apply to the finished work to make it more resistant to dust and dirt?
There are a number of aerosol "fixatives" available in art stores that are commonly used to protect paper drawings. Varnishes are available to protect oil paintings. I recently made some outdoor papier mache Christmas ornaments and sealed them with many layers of marine varnish.
We, in some regions of Venezuela, used to add vinegar and plain W.flour to the paper pulp (mashed newspaper in water) used for crafting Traditional Devil Masks. Even we colour enamel them in the outside, beside its self weight and bizarre forms (horns, protuberances etc) the inside face mus withstand hours of sweat, heat and rather rough movements keeping shape and appareance. After then and once sun dried for 8 hours, we paint the inside surface with a single application of white glue which becomes transparent when dried.
As far as I remember the proportion was, for about two lb. of wetted mashed newspaper, Two tablespoon of vinegar and one cup of weath plain flour.
I hope that, if vinegar smelt doesn't bother at all, it fits to your needs.