As HeavyArty said, really the best thing for representing sand is real sand
It's cheap (free), readily accessible, and easy to apply. Spread normal school glue over your base forms, then springle the sand on. Once fully dru, you can airbrush, and it'll react very nicely to a wash and drybrush, but I would be careful with this as it can EASILY be overdone, simply due to the nature of the surface you'll be working with.