Hello, I have a Coleman pool and we are about to start getting it ready for summer. This year we decided to leave it up thru winter. However, the exterior is very dirty. It has grayish-black staining all over. We have tried hosing it off and scrubbing a bit but no luck. I tried some oxiclean as well and it won't budge. The inside of the pool where the water is does not have these stains. Any ideas as to how to get it off?