I've used that "Kreme" before and it does work, it's a white goo that coats and seals the inside of the tank and gas won't bother it. If your tank has a mild case of rust, you can get by with a cleaning and inline filter.
I cleaned mine by taking the tank off the bike and putting a couple handfulls of pea size gravel in. Then I shook the hell out of it ( like a giant Morraca ). That loosened up the surface rust, and then rinse it out good with a garden hose, blow dry (I used a hair dryer) and reinstall, I never had a problem.
If you store a machine (that has a gastank) for a long period of time, top it off with fuel so the rust won't form.