I have a pretty old pontoon, a 1989 Grumann that I purchased last year and now this year I'm going to have more time to really clean it up. The seast that are in it are very dungy and it looks like mildew or stains on the tops of the seat backs. They are grey and black spots and I really don't know what they are. I tried using regular dish soap but it didn't take it off. Is there any other cleaner that I could try? Any help is appreciated.