Popping dents out of gas tanks is usually the domain of bodyworks garage professionals. However, it may not be as hard as you think. The important thing is to take the proper safety precautions. Home mechanics around the world attest to the effectiveness of the proper method.
Remove Tank And Cleanse
Gas tanks are flammable. When operating on one, there should be nothing around that "could go boom." The vehicle itself has spark plugs and other fire-producing mechanisms, which is why body repair experts often do this work. The gas tank should be removed from the bike to not do more damage to other parts, then it should be thoroughly siphoned and cleansed with fresh water.
Filling the Tank
The tank should be so clean that there is no longer any trace of gas fumes. Hands and clothing should also be gas-free by washing them with warm water and soap. This keeps any gas from getting back on the tank. With a hose, or a funnel, fill the tank with water until brimming. With the cap back on, it's also important that there be no water on the exterior of the tank.
The goal is to freeze the water until it becomes ice, and expands enough to pop the dents out. Once the tank has been in the freezer for a couple of hours, it should be frozen. If a dent remains after multiple tries, a blunted bar--not to puncture a tank--can be used to pop out what's left.