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You can make an empty gas tank safe if you fill the tank with argon or nitrogen and keep the inert gas flowing at a low rate while doing the work. This includes welding and or drilling-cutting.
Personally, I prefer the argon. It is quite heavy and tends to stay pooled in the tank as long as you keep the openings on top, just like if it is filled with water. The nitrogen has a tendency to mix with the atmosphere so requires a higher flow rate.
This method is also much safer than just filling a tank with water as filling with water can leave air pockets with gas fumes. The inert gas method prevents the accumulation of air pockets containing gas fumes.
A couple of per-cautions, you need to have a way to verify that atmosphere inside the tank will not support combustion and that there is sufficient ventilation to prevent the inert gas from flooding the compartment that you are working in.