That's what we are trying to decide just now. I just opened a poll in the low tech forum. (sand vs gravel)
Capping Dirt (Sand or Gravel?) Poll
That sand is a great idea. I have used Pool Filter Sand successfully for years. 0.4 to 0.6 mm grain size. But I have not yet used it over dirt. My only concern is that the grain size is large enough to allow nutrients to fall back into the soil, thus extended its useful life.