We usually put a rain jacket on..
Keep in mind that water is what keeps the sand together.
For most types of sand, a light rain will do nothing to the sculpture, even if it rains all day.
A heavy rain may speckle the sculpture and damage some fine details.
Only very heavy and prolonged downpours can eventually damage sand sculptures.
We also protect the sculptures from wind and rain by a biodegradable windscreen which is sprayed onto the surface of the sculpture after carving is complete.