Your sand painting will last for a very long time if it is put on display so that it doesn’t get rubbed against or scratched. The adhesive itself hardens after exposure to the air. If you would like to further protect your painting here are two ways to do it.
To make the sand itself more difficult to scratch, dab on a protective varnish using a sponge brush. We recommend glossy Mod Podge™ because it doesn’t change the look of your painting when it dries.
Dab (don’t brush) a thin coat over the entire painting so that the surface looks white, but there are no puddles.
Let it dry on a flat surface for about an hour.