Polymeric sand is best to use to fill the joints in between pavers. High quality polymeric sand will not leave any stains on the surface. Gappsi vibrates the pavers while sweeping which allows the send to fill the joints tight and also help the sand harden. Sweeping the excess sand off the pavers before wetting the sand is also very important. This prevents the glue that is in the sand to smear the pavers surface. Most importantly, applying a good urethane sealer will further help the sand to harden and prevent ants from digging into the paver joints and weeds from growing on patios.