Rust in your tank sucks. It can block off your petcock and lead to fuel starvation or worse, it can mess up your carbs. Here is one way to clean the inside of your tank. It is using vinegar. I like this method because it is cheap and gets the job done.

• Remove your tank and empty it
• Remove the petcock to prevent damage
• Plug the petcock hole
• Fill up the tank all the way to the top using regular vinegar
• Close the tank. Ideally you do not use your lid because the vinegar fumes might eat your rubber gaskets
• Let the tank sit for a few days
• Use a long handled brush and brush the inside
• Drain the vinegar. You can reuse it several times, so don't just dump it
• In order to neutralize the acid, mix water with a cup of baking soda
• Dump it into the tank and slush it around
• Rinse the tank with water
• Now dry it thoroughly to prevent new rust buildup (a leaf blower will work perfectly)
• Fill it with new gas and close the tank
• Done