I built a flagstone patio (approx 15 x 20). It is built on a sand base. I'm planning to put patio furniture on it. I'm trying to decide what to use to fill the cracks. Most of them are 1-3". I'm not sure if I want sand because it is close to the pool. Questions: If I use Sand will hit harden enough not to wash away? Or, should I use some kind of concrete or mortar mix? Should I put it down dry or wet? Thanks for any advise.