uh ohhhh

I watched an old guy weld a used fuel tank, he used a shop vac hose in the hole to evacuate the fumes and only welded a small amount at a time. He also did it in a big shop near a large open bay door so the fumes could build inside.
i've read to run a pipe from your exhaust into the tank for a few minutes. it completely purges the tank of flammable gasses.
neil.