Thanks for your question. We always mortar our flagstone into a concrete base. Coupled with the width of raw flagstone joints, there is really no benefit to polymeric sand since the flagstone is held in place by the base. Polymeric sand is best used for interlocking pavers on sand base. We use a custom mix of extra fine masonry sand and mortar mix to grout our flagstone, but installing on a proper base is really the key. Let us know if we can help!