have you had stingrays with that sand? I think, im not a sand blaster or anything, but i would have to thing that the sand used for that has a really sharp or at least square edges on it right? rays would need very soft sand with round edges. i've once heard someone say this, and i've started using it to since i like it so much, pick sand like your gonna rub it on the roof of your mouth when choosing sand for rays. basically what im trying to say is that its gotta be really soft, and i dont think blasting sand is.
