I am working on a friends bike. It has not run in 3 years. He became ill and never was able to get back to riding. Don passed away about 3 months ago and his wife wanted me to take a look at the bike to see what needed to be done. He must have had a full tank when he parked it. It had over half a tank. when i look into the tank it looks good but has a film on it. It wipes off pretty easy. What is the best way to remove this film/gunk? It is on a 2004 Suzuki c50 Blvd.