Cardboard is a versatile material, used for everything from gift boxes to architectural models. Cardboard is inexpensive and readily available. It is easy to cut and many glues can be used to adhere it to itself and other materials. The only downside is that cardboard is susceptible to water damage. This problem is easily remedied by sealing the cardboard with epoxy resin. The resin embeds itself into the cardboard, creating a waterproof material that is similar to plastic. Use basic resin techniques to create cardboard that will withstand long-term use for permanent art projects.
Cut the cardboard into the desired sizes and shapes needed for the project, using a sharp utility knife. Cut the cardboard before sealing it with resin to ensure that the edges of each piece will be thoroughly coated when you apply the resin.
Lay the pieces of cut cardboard on top of a sheet of HDPE (high-density polythene). Use 0.8 to 3.2 mm (1/32 to 1/8 inch) thick HPDE for this project. HDPE is a common plastic that resin will not permanently adhere to, making it an ideal work surface for resin-related work.
Mix the epoxy resin thoroughly in a plastic cup. Do not use paper cups with a wax liner. Epoxy resin gets hot enough during the hardening process to melt the wax liner, which can affect the epoxy's ability to cure properly. The best mixing containers are made from polythene or polypropylene, which resin will not stick to. Plastic food storage containers are an example of this type of container.
Brush the mixed resin onto the surface of the cardboard, making sure to coat the edges of each piece. This will harden the cardboard by filling the voids between the cardboard's molecules with resin. This waterproofs the cardboard and strengthens it enormously. Allow the resin to dry a full eight hours. Flip the resin-coated pieces of cardboard over on the HDPE sheet. Mix and apply another thick coat of resin. Allow 24 hours drying time.
Sand both sides of the cardboard pieces, by hand, with 600-grit sandpaper. Epoxy resin needs to be sanded between applications for one coat to adhere to the next. Wipe the resin-covered cardboard with a rag to remove any sanding debris.
Apply a light coating of resin to one side of cardboard, allowing at least four hours drying time before flipping the cardboard over and applying resin to the other side. The lighter the coat of resin, the faster the initial drying time. This final coat will harden the surface of each piece, keeping any potential moisture from settling into the cardboard. Allow a full 24-hour curing time before applying adhesives to the cardboard. The cardboard is now ready for use in the construction of a permanent art project.
Use a 1-to-1 general purpose epoxy. A 1-to-1 epoxy has a mix ratio that is one part resin to one part hardener, reducing the risk of measuring errors. Any type of cardboard can be used for this project. For large sheets of cardboard, ask a local retailer to save large boxes used for ovens and refrigerators. Cardboard from the back of a notepad is another good source of solid cardboard.