There may be several reasons why you would want to harden aluminum foil. A sculpture made of aluminum foil can be fragile and you may wish to make it more substantial to last longer. Aluminum foil can also be used as an exterior cover for pipe insulation. Heating and cooling pipes are wrapped with insulation to eliminate condensation and to provide a more consistent internal temperature. Aluminum foil is a very good insulator.
Apply multiple layers of aluminum foil to harden it and give it strength. Some applications lend themselves to layering with aluminum foil and some do not. For those that do, it is a very good option.
Brush resin onto the surface of the aluminum foil using a small paint brush. Make sure you cover the entire surface with the resin. Apply several thin coats of resin and let each one dry for at least 12 hours before applying the next one.
Cover the foil with pipe plaster using a small paint brush to apply the material. Pipe plaster is a good option for foil used to insulate pipes, but can also work for an aluminum sculpture if it is being painted or you don't want the foil to show.
Spray the foil with spray paint to give it more structure and visual appeal.