Things you will need:

Super Glue
Some sort of sealer (to waterproof the paper)
Reinforcing substance (paper mache, plaster of paris , artistic gel, fiber glass)

Now for just a little basic explanation. For Halloween I have been making skull masks with the use of papercraft for my girlfriend and I. Through trial and error I have discovered a cost effective and easy way to reinforce most papercraft creations. The one issue I manly had was that most of the substance I used to reinforce my mask soaked straight into the paper; warping it and causing some of the glued tabs to come undone.

This is just a basic guide to put you on the right track.

Step 1: Sealing the Paper

Now there is no exact place to start with the sealing process as it depends on the papercraft model. For me I built my skull mask first then I started the sealing process.

First you want to apply super glue to all your connecting tabs and folds so they will become rigid and wont fall apart. Be careful of the superglue fumes and accidently spilling it on yourself.

Once the super glue has dried (takes a few minutes) apply the sealer to fully cover all the surfaces of your model.

I used artistic gel to seal my mask (really not the best as it can cause the paper to warp). Try using some sort of sealer that will be friendly to the paper and not soak it causing damage, like acrylic sealant (Please comment if you know of any others).

Step 2: Reinforcing

Once your model has been completely sealed you can now begin the reinforcement process, I suggest that the model be fully constructed as it may be difficult to properly attach the reinforced pieces together and make sure they hold.

Note: I would advise that you seal a separate piece of paper and test your reinforcing material.

When choosing your material it is advised to do some research, to see what would better suite your purposes (if you have any suggestions for reinforcing materials please comment them and give an explanation please).

Step 3: The Rest Is Up to You

Now you should have a reinforced paper craft model, but there is still more that you can add which is up to you and your creativity. Above you can see my finished masks that were a hit for halloween.