I am in the process of draining and figured id give everything a good rinse down with the pressure washer. To my surprise even after a thorough spray down I am left with what almost looks like sand spots? I tried using a metal bristle brush on it too but it still doesn't seem to come off. Can anyone help me identify what exactly is going on here? I am using one of the gentler nozzles on the pressure washer as I was worried about damaging the plaster. How do I clean this mess? 