We installed a flagstone patio for a customer, and put a walkway from the main patio down to the walkway around the yard. There is about 8 inches of fall on a 12 ft walkway. We put an aggregate base down, then mason sand and filled the cracks with paver locking sand (the new kind that sets up a little). The client just emailed me as said when they get a big rain, it is washing the sand out. short of filling the cracks with mortor mix, is there something else I can fill with to stop the errosion?