I used the high performance (best quality) sand to fill the cracks in a
flagstone patio. (2 inches sand underneath). It is suppose to be like
cement when dry and hard rubber when wet according to the adds. Wrong!
When it rains it turns to sticky mush that can easily be dug out by a
child's toe when just walking. When dry it is hard, but it keeps
flaking off bits of sand that is constantly tracked into the house. And
yes I went by directions and I had 4 days of low humidity and 90 degree
heat for curring.
In short, you will regret it if you use polymeric sand for your
flagstone joints. mes.