Hardboard is a classification given to manufactured wood. Hardboard takes the fibres from wood and presses them together to form the hardboard. Manufacturers use wet and dry processes to produce hardboard. The product typically comes with a smooth and a rough side as a result of the manufacturing process but can be made with both sides being smooth. Cutting hardboard requires careful planning and very sharp tools to avoid splintering the wood. Gluing the wood follows normal processes with a few additional steps.
Pour some wood or polyurethane glue into a small paper cup. Add some water to dilute the glue. Mix the solution with a paintbrush.
Apply the diluted glue to the hardboard sheets where you will be joining the wood. Use the paintbrush to evenly spread the glue over the entire surface. Allow the surface to dry for the approximate time listed on the bottle of glue before joining the pieces together.
Apply the undiluted glue to the surface.
Join the two pieces together and attach clamps to hold them firmly together. Allow the set-up to dry for at least twice the time listed on the bottle.
Hardboard is very absorbent. The above process allows for the absorbent nature of the wood. You can use an industrial adhesive, which has a lower water content than glue.