Begin by painting the foam not covered by sand with paint arcylic using a brush to prevent it melts reacting with spray paint.
Once the entire surface is covered with sand and the glue is dry, find a good place to spray the aerosol paint.
A place outside well-ventilated but draft-free is ideal for painting with spray.
It is best to use spray paint to paint the sand because it goes faster and it helps to stick the smallest grain of sand.
The result is very convincing, it will be a very good base color for shading.
Despite the use of a lot of glue, the foam board rolled up. A large brush was used to make the final touches.
Here is the model with the base coat applied all over it.