Answer: Sand helps lock the pavers in place but it does tend to wash away after a time unless it is compacted at the time of installation. There are mechanical compactors that you can rent to help tamp down the sand and make it less likely to wash away. You have two basic choices when choosing a joint sand for your paver project: an ordinary coarse sand or a coarse sand with a chemical binder. Ordinary coarse sand has been used for many years and will perform well. It's also less expensive than the alternatives. However, there can be washout with rains, weed seeds can land between pavers and germinate, and ants can carry joint sand away.
The alternative to builder's sand is "polymeric sand". Polymeric sand is just like any other sand, except that it's been impregnated with a chemical binder that hardens the sand. That binder will either be organic or man-made.
Whatever your sand choice, simply sweep the sand into the joints between the pavers, being careful to completely fill all joints to the top of the paver surface. Once swept, the entire paver area needs to be compacted to both settle the sand and to seat the pavers into the crushed stone base you prepared earlier. Be sure to protect your pavers during compaction by using a pad of some sort to cushion the compactor's plate against the brick. Otherwise you'll likely have significant scuffing and possibly some cracking (this tends to happen more often with clay pavers). We use a urethane pad for our compactors, and you may be able to rent one from your local rental yard.
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