Welding sump to fuel tank

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I have a steel fuel tank that I am going to be welding a competition engineering sump on to.  I think its mild steel.  It doesn't involve much more than drilling 5 holes for gas to flow down into the sump, and then pulling the paint off where it needs to be welded, and welding it.However I think the sump is mild steel and I am sure the tank is mild steel.  I have a bunch of 309-L rod, and I do have some mild ER70 as well.  I prefer using the 309-L as I've used it more than ER70 and I feel I am better with it.Does it matter if I use 309 or ER70 since its mild anyway?Also, I have had the tank in the shed for about a week now venting any fumes.  I am thinking about washing it out with water and soap to be safe, but I don't want to induce any rusting.  Any suggestions?
Reply:venting a used gas tank for a week, month, year is no guarentee it'll be safe to weld!washing it out with soapy water will not guarentee it's rendered safemy suggestion...make or buy a swirl pot and plumb it in.
Reply:Hrm.  Should I take it to a welder and have him do it?  I dunno, bleh.
Reply:Call around and ask for shops that have done it. It's better to pay a shop a few bucks than to either lose your life or start a fire and burn down your garage. Me - most times I just buy a new tank.John -  fabricator extraordinaire, car nut!-  bleeding Miller blue! http://www.weldfabzone.com
Reply:This is for a BMW - tank is about $900.  I would just put a fuel cell in.  Almost leaning toward that...But that ruins most of my trunk - someone's got to be able to weld this
Reply:Contacted someone local (finally, no one answers their phone!) and it looks like it'll be a little over $100 to get it done.  Not cheap, but I need it done.
Reply:Originally Posted by Jon KContacted someone local (finally, no one answers their phone!) and it looks like it'll be a little over $100 to get it done.  Not cheap, but I need it done.
Reply:To weld a fuel tank you need to 1.make damn sure it's dry 2. make sure there are no odors 3. purge/fill the tank with Co2 or another inert gas before you weld/while your welding. Also once your done look for a product to coat the inside of your tank to prevent rusting.