I've just finished making one (calico cloth, caulk, sand added on top, thin layer of watered down PVA, paint). The sand I used was "sharp sand" which has more little stones in and I've found that some of the larger ones of those are tending to come off - not in large quantities, nor that you'd particularly notice. The sand itself is absolutely rock-solid. The whole thing rolls up easily with no problems.
Next time, I'll use silver sand for most of the cloth with just some patches of sharp sand for variety.