Another diesel tank welding question

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Hi! My first post here, found what looks to be a really good forum by googling my current predicament and reading some threads on here.So I have this diesel fuel tank from a tractor.  I had to cut a hole in the top(used a cut off wheel) to get to the inside.  Now I need to weld it back up.Its a steel tank, about 10 gallon capacity, for the most part its a cube with a curved top and bottom.From other threads on here I have ruled out:*Filling it with fuel to reduce oxygen level- I dont care what anyone says, Im not doing it..*Purging with exhaust fumes.. Not sure how I would do it in this case even if I wanted to.. theres only one hole big enough to accept exhaust fumes, and they contain oxygen anyhow.*dry ice pellets.. due to the location of a supplier and where the work is to be done, not practical.My plan(with some gaps in it).1) Clean it out with room temp water(no hot water near by) and a good amount of Dawn.  I had half a mind to do a first pass with mineral spirits prior to the water and Dawn, but not sure that would accomplish anything.2) Position the tank so that the hole I need to fill is on top.  Drop a hose from my MIG's Ar/CO2 mix into the filler neck, seal with tape, then poke a hole in said tape.3) TIG weld the removed peice back in.I guess the part that has me a bit confused/concerned is keeping the argon in the tank?  Seems like the repairs I read about are for a big tank.  This is a 10 gallon tank, and the panel I need to weld back in is about 6x6" so its a fair amount of opening.  Has me wondering how to control the purge flow.Thanks!ScottMillermatic 150Lincoln Precision TIG 185Lincoln TombstoneHypertherm Powermax 30
Reply:Your plan sounds good, but why are you not using pure argon on your tig? Take some tape  and tape the 6x6 plate in place except for the 4 corners, let it purge for a minute and tack those spots, then remove the tape and clean up the residue and finish. You could always tack some tabs on the 6x6 so you can simply set it where its going without it falling in.
Reply:Hole on top? Fill it up with water & weld it up
Reply:I'm with old skool, fill the tank with water
Reply:If you used a cutoff wheel to cut out the piece you would have sent quite a few sparks into the tank?? Was it empty then?
Reply:I welded a diesel tank from a tractor. I just drained it well. No washing I then used low pressure compressed air about 10 psi to purge so fumes couldn't build up while welding. No flash no boom just weld.Millermatic 252millermatic 175miller 300 Thunderboltlincoln ranger 250smith torcheslots of bfh'sIf it dont fit get a bigger hammer