This is an interesting question, taking into account that a lot of people don't consider painted paper models as "true" paper models.
I experimented a little bit on how to best prepare the paper for the painting and I guess weathering process later on. In my experience, after finished with the model building, I use an acrylic primer such as Vallejo in a spray can, which allows for a thin coat of water-proof layer. Usually, applying three coats should be enough. After that you can pretty much do as you wish - I use Revell Aqua Color (acrylic) and basically treat the paper models as a plastic models.
Also, take a look on the following thread where a lot of people helped me with the same topic:
Painting paper models