Things You'll Need
Cardboard craft furniture, decorations, and even "pepakura" -- or cardboard armor for cosplay events --can be preserved by hardening your cardboard projects. There are a few methods available to accomplish this: Create a shiny surface thorough the application of PVA glue -- basically white "school" glue used in craft projects. Or you can go all out with a coating of fiberglass resin to harden and waterproof cardboard.
Apply a PVA Glue Coat
Mix one part PVA glue with one part water. Spread out newspaper for your work area.
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Apply the mixture to the cardboard surface. Allow it to dry to the touch -- about two hours.
Apply a second coat of the PVA mixture. Take the time to apply a coat to the underside or inside of your cardboard project too. Allow it to dry to the touch.
Mix up two parts PVA glue and one part water.
Apply a third coat using the more concentrated PVA glue mixture. Allow the surface to dry to the touch. You should notice the cardboard is also taking on a shiny, plastic-like appearance.
Create another mixture of one part PVA glue and one part water. Apply it to the bottom or the interior of the cardboard project. Allow it to completely dry.
Mix up a final coat of two parts PVA glue and one part water. Apply it to all surfaces of the project. Allow it to completely dry. You should have a shiny, hard cardboard project.
Apply a Fiberglass Resin Coat
Prepare your work area in a temperate, well-ventilated room. Spread out the newspapers. Place the silicone mat on top and rest your project on this mat. It is less likely to stick to the silicone when the resin hardens. Put on safety gear, as fiberglass resin can get messy and is difficult to remove.
Mix the polyester resin with the catalyst. The amount you need depends on the size of your cardboard. However, resin sets in less than 17 minutes. It is preferable to start with a small amount and mix more later if needed. Mix 12 drops of resin catalyst per ounce of polyester resin.
Brush the resin mixture onto the cardboard surface. You may need to complete this project in two parts if you cannot brush the bottom or inside too. Apply enough resin for it to fully penetrate the cardboard.
Allow the resin coat to cure for between six and 24 hours. Apply a second coat of polyester resin, if you desire. At this time, you can also coat the inside or the bottom of the cardboard with a layer of resin for complete protection. Allow that coat to cure for up to 24 hours.
Some crafters also use a few coats of polyurethane to harden cardboard. If the polyester resin dries to a tacky finish, brush on cornstarch powder. Within a few hours, it should lose that tackiness.
PVA glue coats are not waterproof. PVA glue coats may also crackle or shatter if the piece of cardboard flexes.