Welding Tips and Tricks

Greetings , I am having porosity problems when mig welding thin steel containers such as fuel tanks. If I cut out a rusted area and tig repair it there is seldom a leak. When I mig weld in a repair the welds have leaks all over. I have not been able to pin down the issue - wire, welder, tecnique , etc.. I was told to weld as cold as possible & start off of the joint. Any advice here would be greatly appreciated.
Thanks

Welcome, John!

A question for you: "If I cut out a rusted area and tig repair it ": when you MIG, are you also cutting out rusted areas? In any event getting down to bright shiny metal is probably best. What kind of wire are you using? I did a search and found one or two references to the additional deoxidizers in er70s6 wire. I wonder if that would help.

Artie,thanks for the quick reply. I am using er70s6 wire. It welds great- when it comes to joining two pieces of metal it's fine. But when welding something that has to hold pressure there are gobs of porosities. I have talked with welder salesmen, welding supply salesmen, as well as professional welders and have found no " here's what your doing wrong " answers. I don't mind Tiging the patch panels in, it's just so much more time than Miging. There must be a wire or a tecnique out there that would allow welding thin vessels without porosities.
Thanks

John,
What gas are you using? On thin gauge metal I would suggest using a 95/5 Argon/Co2 blend instead of the often used 75/25. See if this clears up some of your issues.

There are others here with way more Mig experience than me and hopefully they can pinpoint your problem. Pictures of the offending welds would also help identify your problem.

There's no doubt that light weight vessels can be Mig welded, it's done everyday and I'm quite sure somebody here knows the answer.

Len

John,
Welcome to the forum! I am a little confused by your post, lets see if we can get a clearer image here. First, as was mentioned, a picture is worth 1,000 words so that would be the best help. Second, it seems like you are welding on used fuel tanks which brings to mind that you might be getting the porosity from whatever was used to clean the tanks or the actual fuel vapor is igniting causing porosity. Third, you did not mention the material thickness of joint configuration, this will help as well. I know what they mean by as cold as possible so as not to just blow through the material but you do want proper depth of fusion, not just a weld sitting on top there. Your S-6 wire is fine for the application as far as I can tell. What I don't understand is why you would get porosity with mig and not with tig on the same joint? That does not make sense to me. Normally it would be the other way around.
-Jonathan

Len & Jonathan , thanks for the replies. I have not tried 95/5 or straight argon but I easily could on the next project. I don't have any pics but will start taking them for reference. The metal thickness can vary from .032 to .075 depending on application - automotive, AG , truck step tank , etc. We hot tank & steam clean , then acid treat to remove rust. Re hot tank to neutralize the acid & steam again. Tanks with baffles may need access holes cut to allow steaming & inspection. Welding the patch in is where we see the porosities.
Thanks for your help.