I have an inground pool with a vinyl liner with an Aquarite SWG. I recently noticed that there were brown stains around the screws that secure the skimmer opening to the pool sidewall. I removed a couple of the screws to try to remove the stain but it did'nt come off. What would cause this staining and what can I use to remove it? What is a good all around cleaner for the steps and liner to remove dirt?