Safteying a gas tank

Why in the world would you want to do this?

There are ways to prepare a tank for a patch like this but since I have no idea how thorough you are in your work, or anything else about you, this is one repair I'd never suggest a methodology for. Explosions from gas tank repairs are all too common. You have to rid the tank of all of the gas fumes and that's much harder than it would first seem. If you decide to go about this, then at least spend some time reading up on gas tank explosions.

Tell us more about what you are doing. Nobody needs to repair a tank like that. Get a new tank or have it repaired by a professional. Even a hard to find tank can have a liner put in it to end all leaks. This is either a hoax or you are so far off the wall that you are in a danger zone beyond your knowing.

After removing tank, Start out with a couple gallons of fresh gas, slosh it around, and pour off for later use. Stale / stabilized gas evaporates * very slowly * Fresh gas evaporates fast.

Place tank in sunny location, remove cap, hook a long hose to fuel feed, pump Nitrogen or Helium into tank to displace fumes, do a little at a time. Compressed air will oxygenate the fuel, and turn the tank into a bomb. When liquid is removed, oil up sender unit, apply shaving cream to bolts [ eliminates spark hazard ] Once sender is out, air can enter baffled areas. and tank would be much safer. Replace sender, tape up holes, and give a second blast of inert gas.

A Nitrogen outfit, Small bottle, cheap torch [ on / off valve + hose ] Regulator could be bought A CO2 outfit would be cheaper, but I don't like CO2 for purging, trapped moisture not found in inert gas

Failing that, a party balloon kit [ Helium ] can be had cheaply, you get a disposable cylinder of Helium, hose and balloons

Why not take tank to radiator shop, where they just dunk it in hot, acid treated water, prior to soldering? Don't use your garden hose on tank, environmentally uncool !

Make a video when doing this so we can watch in on the news.