You can't count sand unless you specify a unit to count, as you did by saying "grains" of sand. You can't count "sands" because it's unclear how much sand counts as "one sand." Is it grains? Molecules? Grams?

Keep in mind, units can be implicitly rather than explicitly stated. If you ask a waiter to bring two waters to the table, they'll understand from context that you want two glasses of water.