Build a mold of your mask out of newspaper papier-mache or foam. Craft the shape of the mask by pressing the paper when wet or carving the foam. Use blown-up balloons as necessary to frame curves in the mask if it has fairly rounded sections. If you're using papier-mache, let the mold dry hard. Whichever mold material you use, keep the inner mold surface as smooth as possible.
Clean the inner surface of the mold with acetone and a sponge to remove any contaminants. Don't use this approach with papier-mache -- it will destroy the paper. Instead, just brush it clean with a soft brush. Spray the dried or finished mold with a nonstick wax to keep the fiberglass from sticking when it hardens.
Pour resin into a paint tray. Cut various pieces of fiberglass cloth into 3- or 4-inch squares to position them for application. Use a paint roller and paintbrush for the resin application. Mix sufficient hardener into the resin and begin the layering process. "It's a percentage system, so if you use a quarter of the resin, use a quarter of the hardener," advises a tutorial on Paper Wings.
Apply the first brushing of resin to the mold surface. Lay down a complete layer of fiberglass cloth on the resin. Press the cloth into the resin with a brush or roller to squeeze out the air bubbles. Paint more resin on top. Apply at least two or three layers and then let the first application dry and thicken. Sandwich more resin between every two layers of cloth.
Apply additional layers until your mask is at least 20 millimeters thick. Apply the final cloth layer and cover it with resin. Press out any air and finalize the resin application. Let the fiberglass cure and harden. Clean up your work area in the meantime and close up all product containers. Use turpentine to clean up spilled fiberglass resin if it lands somewhere other than on your mold.
Allow the mask to cure and harden for at least a day or two. Check the curing process by touching the mask in the mold after a few hours.
Carefully pry the mask from the mold, using a plastic spreader to separate it. Put on a respirator mask for protection. Begin sanding the surface of the mask to smooth out any jagged areas or lumps. Use an automated sander to trim and smooth the edges of the mask quickly.
Wash off the sanded mask with water and let it air dry. Examine your handiwork and paint and finish the mask as desired.