welding a diesel fuel tank?

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I was just wondering if anyone had any suggestions.....  I've cut out bad spots on a bulldozer fuel tank and welded in metal patches.  The tank had been drained and the metal was cleaned up (paint etc.). The metal is about 1/8" thick.  I used a mig welder instead of a tig because of limited space.  I was very thorough on running a seal weld around everything and then a cover pass was put on over them.  I filled the tank partially to test for leaks, and it seems there are 3 to 4 leaks and they seem to be beside the welds???  Any input on this??  Am I doing something wrong?  Thank-you
Reply:Grind it off and fix it.  I do some welding for heat exchangers and they have to be air tight.  Tig or pulsed spray work great, I get a few leaks with mig, so I don't use it anymore for that application.DavidReal world weldin.  When I grow up I want to be a tig weldor.
Reply:I hope you didn't fill it with diesel and now have to repeat the cleaning to did it of diesel. Next time fill it with compress air or nitrogen to no more than 2 psi. Check for leaks with soap suds. This will save you a lot of time if you have to do any rework.
Reply:I always had real good luck repairing them with 3/32 6010. No cleaning and undrained sometimes. Anything from truck fuel tanks to one train engine. Never got any of them back and they were warranted.The difference between art and craft is the quality of the workmanship. I am an artist.
Reply:Originally Posted by baclacI was just wondering if anyone had any suggestions.....  I've cut out bad spots on a bulldozer fuel tank and welded in metal patches.  The tank had been drained and the metal was cleaned up (paint etc.). The metal is about 1/8" thick.  I used a mig welder instead of a tig because of limited space.  I was very thorough on running a seal weld around everything and then a cover pass was put on over them.  I filled the tank partially to test for leaks, and it seems there are 3 to 4 leaks and they seem to be beside the welds???  Any input on this??  Am I doing something wrong?  Thank-you
Reply:Mig is a bad choice for this because there can be no contaminants at all. As soon as you start welding you get smoke and the smoke contaminates the weld, the contaminants inside the tank will also be drawn into the weld when you start melting the metal. The same goes for the tig for the same reasons. Neither one has the cleaners and oxidizers that the flux on a stick has. 6010 works well in an oily, greasy, scaly, painted or just plain nasty situation. Sometimes you just can't get rid of all the crud.The difference between art and craft is the quality of the workmanship. I am an artist.