cleaning tank

[Jamtndll]

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.