Cardboard is useful for building and storing things. Sometimes cardboard is used to make something that a person wants to last, such as a special art project or useful home-improvement improvisation. However, cardboard is made of paper, which makes it susceptible to humidity and other water damage. Here are a few tips for how to waterproof cardboard.
Seal the edges of the cardboard
Seal the edges of the cardboard with duct tape. Leaving the edges of cardboard exposed will allow water to seep into through the porous sides of the cardboard even if you waterproof the rest of the surface area. Make sure to cover all exposed cardboard edges.
Paint or spray the cardboard with a waterproof coating
Spray the cardboard with an automotive primer and then let it dry. You can also use a variety of waterproof varnishes to provide this initial waterproof coating for your cardboard. Since you will be painting the cardboard with another layer of protective covering, make sure the substance you use for this initial layer can be painted over.
Next, paint the cardboard with an outdoor latex paint similar to what you would use on the outside of your house. Having these two layers of waterproof sealant--a primer and a latex--should keep your cardboard stiff and dry for quite a while.
Cover your cardboard with polyurethane or glue
If you are open to waterproofing your cardboard through other means than layers of paint, you can coat it with a variety of polyurethanes or glues. Because these products are waterproof, they provide a waterproof layer when applied to cardboard. However, it is hard to paint over them and they may not last long, especially if the cardboard is exposed to weather such as rain and snow.