I have had issues in the past with the upholstery in my Supra boat. I was told I could not use anything other than water to wipe the cushions down with, or the cushions would get hard and crack. However, somethings don't wipe off with just water. What do you suggest when something such as mildew gets on the seats and after a 10 minute wipe down with a lot of elbow grease and wet rag does not do the job? Any ideas?