Why does my coated surface have hard crystal like pebbles in it?

The resin may be crystallized from temperature shock. Warm the resin by placing the closed container into a warm bucket of tap water for about 20 minutes, If the crystal is still visible repeat process. Sand surface lightly with 200 to 220 grit sandpaper, clean with denatured or rubbing alcohol using a soft lint-free type cloth to remove dust or sanding debris. After alcohol dries you're ready to re-flood your project.