What is the MOST effective manner to produce a hard surface on a cast?

Using as little water as possible during the mixing process is the most effective manner to produce a hard surface on a cast. This is because the consistency of the mix directly impacts the material's density and strength. A lower water-to-powder ratio leads to a denser mixture, which results in a harder setting material once it cures. When excess water is introduced, it can create voids and weaken the final structure, making it less durable.

Additionally, while some additives like borax or calcium tetraborate may have specific benefits, they are not as directly influential on the hardness of the cast as managing the water content during mixing. Balancing the right amount of water ensures that the casting material has optimal properties for strength and surface finish.