After laying porcelain pavers in samd in my new lanai, we then used polymeric sand for joints. it looked great until about six months later. i started to notice some joints were developing rust. i then proceded to remove the polymeric sand. i left all joints free of anything for two to three weeks. then i started to see it again. just a little less. could it be that the sand base was not completely dry? i dont know what do use at this point.