Cardboard is highly versatile. Manufacturers make boxes, tubes and all kinds of containers from cardboard, while model-makers prize it as a cheap material that is easy to cut, bend and shape. However, if it gets wet, cardboard quickly soaks up moisture, loses its rigidity and disintegrates. You can easily waterproof cardboard by treating it with paraffin wax, a product obtained from petroleum and used in the manufacture of all kinds of waxed papers and cartons. Paraffin is so good at waterproofing, it was once used to seal the Egyptian obelisk known as Cleopatra's Needle that stands in Central Park, New York.
Put a white household candle, which is made of paraffin wax, into a plastic bag. Tie the mouth of the bag closed. Put the bag on a hard surface and wallop it with a rolling pin, to break up the candle into small pieces of paraffin wax.
Open the bag and tip the broken bits of candle into a clean tin can. Stand the tin can in a small saucepan containing about 5 cm/2 inches of water. Put the pan on the stove over a gentle heat to melt the wax in the tin can. Heating in this way rather than directly in the pan stops the paraffin wax from burning, or leaving scorch marks on the pan.
Allow the water to get hot, but do not let it boil. The melting point of paraffin is around 44 to 66 C/120 to 150 F, well below the the boiling point of water. Top up the pan if the water level falls.
Spread newspaper over a work surface to protect it. Place the cardboard you wish to treat on the newspaper. Put a paintbrush and a ceramic plate next to the cardboard. When the wax has all melted, carry the saucepan to the work surface and stand it on the ceramic plate. The plate will stop the hot pan damaging the work surface.
Dip the paintbrush into the hot wax and apply a coat of melted paraffin to the cardboard. Work the paraffin wax into all areas of the cardboard, including the edges if it is a flat sheet, or the joins and seams if it is an object such as a carton. To make a good waterproof seal, make sure the cardboard is completely saturated with wax.
Allow the wax to dry, then turn over the cardboard and treat the reverse side if it is a flat sheet, or the inside if it is an object such as a carton. Allow the wax to dry again, then apply a second coat. You may need to return the saucepan to the stove from time to time to keep the wax liquid. Let the second coat dry to leave your cardboard shiny and waterproof.
Paraffin catches light and burns readily, so keep it away from naked flames.