Hard coating polystyrene with waterproof PVA and kiln dried sand can be done using the following steps:

1. Clean the polystyrene surface thoroughly to remove any dirt or debris.
2. Apply a layer of waterproof PVA glue onto the polystyrene surface using a paintbrush or roller. Make sure the layer is even and covers the entire surface area.
3. While the PVA glue is still wet, sprinkle sand over the surface. Make sure the sand covers the entire surface area and is evenly distributed.
4. Allow the PVA glue and sand to dry completely. This can take several hours, depending on the thickness of the layer.
5. Once the glue and sand are completely dry, use a soft brush to remove any excess sand from the surface.
6. Apply a second layer of waterproof PVA glue over the sand layer to seal it in place. This will also help to waterproof the surface.
7. Allow the second layer of PVA glue to dry completely.

Your hard-coated polystyrene is now ready to use! It should be resistant to water and more durable than uncoated polystyrene.