There are two ways to make cardboard boxes waterproof. The first method, coating the box with an acrylic elastomeric roof coating, is simple, but the waterproofing will not last much longer than one year. The second method, wrapping the box in a covering of fibreglass, is much more complicated, but it will last for many years. Your circumstances will determine what kind of waterproofing you want to place on your box.
Place a clean cardboard box on a tarp outdoors on a calm, windless day. Make sure the sides of the box are free of oil, dust and dirt; if the box is dirty in any way, choose another box to use.
Remove any tape or staples holding the box together. Keep the box in its original shape, so you can paint the inside and outside of the box, but keep the box flaps open. If any part of the box is closed, the coating will not reach all sides and edges of the box.
Mix the acrylic elastomeric roof coating with a paint stirrer. Make sure it is creamy and bubble-free.
Apply the roof coating all over the box with a paintbrush, starting with the inside, and then covering the outside. You will not be able to get all sides of the box in one attempt, because you have to set one side of the box on the ground.
Wait for an hour or two until the paint dries. Turn the box over and paint the parts of the box that you missed.
Apply a second coat of paint to the box. Pay careful attention to any box edges or any places that you missed before. Allow the box to dry for 24 hours before closing the flaps and using it.
Set a cardboard box with a separate lid on a tarp. The polyester resin will harden to a finish that is impossible to bend without breaking, so boxes will flaps will not work for this waterproofing method.
Mix the polyester resin. This material comes in two parts, a resin and a catalyst. Mix 5 drops of catalyst for every 28.4gr. of polyester resin.
Spread a thin layer of the resin over the cardboard box using a paintbrush.
Lay fibreglass deck cloth over the cardboard. Brush any wrinkles out with more resin.
Place more layers of cloth around the corners and bottom of the box, painting an additional coating of resin over each new layer. Overlap the edges as you lay down more cloth. Five layers of cloth should be enough to provide an effective waterproof barrier.
Paint one final coat of resin over the entire cloth-covered box.
Cut any cloth hanging over the edges of the box with scissors. File any ragged edges with a nail file.
Paint the edges and surface of the box with plastic filler. Use a squeegee to smooth the edges of the box.
Allow the box to dry for 24 hours. Rub the entire box with sandpaper to create a smooth finish.