new steel to old rusty-mig question

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heres one everyone probably battles every now and again. We are in the process of lining some old square tank floors with new sheet steel. The old floors leaked from the rise etching into the some of the flow areas under the filters. These are fairly big tanks requiring a few sheets each and about 140' of weld. Clearly the welds need to be tight ! My problem is that after days of prep grinding and wire wheeling, now that the sheets are in we find there is just too much corrosion still there (even though it looks pretty good) to be able to get good clean welds. This is a big issue !  I am forced to go ever the welds when done with a sealer just to be sure we fill any pin holes. They are open top tanks and no way to pressurize them to check them, they are also in a pretty nasty area to work with little ventilation. I am trying to avoid using any flux process to keep the smoke to a minimum - the air gets thick in just minutes. so I need advice on better wire to run ? should I deal with smoke and go to a dual shield - which I do not want to do.   Or can someone tell me of a good weld sealer that does not have silicon or silica in it ? cant have any silicone according to the company that owns them. My father who has been long retired and out of touch for a long time says locktite used to  make a product just for running over weld areas to seal them ? I have used Lexel on rivited sheet 304 seams before and that worked good between parts but not sure about over welds. I am pretty desperate here, we need these tanks water tight by the end of this week ! please help and if you know where we can get the product online or other please advise.   Thank you in advance.
Reply:Hi xcflyn, and welcome!My father who has been long retired and out of touch for a long time says locktite used to make a product just for running over weld areas to seal them ?
Reply:could try running a torch across the weld area and then wire brush again.
Reply:Since you didn't bother to mention what the tanks contain (oil, petroleum, water, etc), it makes recommending a sealer/process very difficult.If the contents are not caustic, you may want to look at using a marine sealant called 5200.  Dang stuff will bond to and seal nearly anything.  It's a polyurethane sealant made by 3M.Syncro 250 DX Dynasty 200 DXMM 251 w/30A SG XMT 304 w/714 Feeder & Optima PulserHH187Dialarc 250 AC/DCHypertherm PM 1250Smith, Harris, Victor O/ASmith and Thermco Gas MixersAccess to a full fab shop with CNC Plasma, Water Jet, etc.
Reply:you can still hydro test, just fill the tank with water and check for leaks. doesnt have to be presurized. depending on what the tank holds depends on what you can seal the tank with if needed. i've cladded ALOT of tanks and never had to seal any of the welds. if its still bad can you put in more material? i've always had to stick weld all the tanks i did. how tall is the tank? is there an access port or manhole? you can tie off box fans above you to suck the smoke out of the tanks. or if you want you can buy a smoke eater fan. its a portable squrrle cage fan with flex tube running to or from it to suck out the smoke. if there is no manhole where you can put a fan for fress air, you might be better off sticking it anyway. useing c25 gas or any combo gas can result in a bad thing in a confined space. argon will stay low and deplete the O2. its heavier than air and can kill you if exposed long enough. say passing out because of it. what ever you decide to use, make sure someone is OUTSIDE of the tank and able to call for help.
Reply:Thank you all,  I will try to address a few of the questions that came up that I can remember.  The tanks are open on the top, approx size 14' X12' X 3' high, problem is they are inside another structure that surrounds them.To fill them and find the leaks would be difficult because of the double floor single wall design. The floor leaks at a wall but then drips out of the old existing in a different place, sure you can see how you would not find the original leak area. They are technically water tanks running RO water but has a etch of some sort in it. makes it a little corrosive on some things, like silicone but not on fiberglass or rubber or some other elastomeric type stuff ? I know devcon will hold up but its not really made to go over as a sealer (but we have repaired radiators that were unweldable  with the stuff).  the torch trick I did try but the corrosion is pitted and just so embedded in the old tank material it wont come out until you dont want it too-while welding. I will try to research some of the sealers mentioned. I worked in the tanks again this morning - running multipass's in some of the poor areas, I have seen good looking welds leak, and alot of these look like they are marginal, I believe a sealer will be the answer! Thank you again.
Reply:Not a profesional in any way, but the first thing that popped into my mind was POR 15we used it extensively in autobody to keep water out of welded areas & stop rust in its tracks, coating floor pans & trunk floors.we used it to treat many motorcycle gas tanks, I have seen it used many times to fill pin holes instead of welding to keep paint original on some old bikes. & seen it used a few times when after welding a broken mount on a gas tank & leaked after the repair instead of re-welding to be coated with this stuff. or just because the owner didnt want to repaint his tanks, & tried this first. it works well at this & really bonds to rusty metal!1 shop I know coats every tank they sell & every motorcycle they restore unless customer specifies otherwise. & it holds up well with gasoline.here is a link to their site.http://www.por15.com/POR-15/productinfo/1GB/RandyLast edited by knucklepanshovel; 07-25-2010 at 08:03 PM.Miller 140Campbell Hausfield migVictor torch outfitBlue Point torch setSmith torch with vintage Airco regulatorsVictor type A torch setHobart pipelinerLincoln Ranger 9Lincoln 225 tombstone
Reply:I'm still of the mind to recommend flux-core, and run a fan. There is no loss of shielding gas with a fan running, so clean air is just a blower away. I would be more concerned with the welds rather than a sealant. Stops and starts will be the culprit with mig; it always cold starts unless you have an advanced machine. proper grinding at stops/ start junctions, and proper backfilling can fix these issues you are having I suspect.After that, then look to Sundown's question- what do these tanks hold???(EDIT) Re- Read your post. RO water is too pure for it's own good, and will draw minerals back into it because it is negatively loaded as it were. The RO water WANTS to take coppers, zincs, iron from anything it touches, to make the water complete again. SEAL the tank. My best idea is to look into the epoxy that some shops reline household water pipes with. It seals leaks, and lines the pipe to prevent future leaks. That's my best guess!! Good luck brother.Last edited by Rojodiablo; 07-25-2010 at 11:21 PM.And then, after so much work...... you have it in your hand, and you look over to your side...... and the runner has run off. Leaving you holding the prize, wondering when the runner will return.
Reply:Originally Posted by xcflynThey are technically water tanks running RO water but has a etch of some sort in it. makes it a little corrosive on some things, like silicone but not on fiberglass or rubber or some other elastomeric type stuff ?
Reply:If you can get a piece of plexi glass approximately 2' x 10" and get foam around the perimeter  to create a seal and then find a way to connect a shop vac to it you have a vacuum box.  We use these to test the welds for leaks on the tanks we do work on but ours use a venturi powered by an air compressor.  You spray the weld with soapy water then place the vacuum box over the weld then apply the vacuum.  Also on the tanks we work on the floor extends pass the tank walls and those welds are tested with a dye penetrant that glows when exposed to black light. The inside perimeter is welded first then the dye is sprayed along the outside perimeter then the inside is checked for leaks. Once all repairs are done and retested until no more leaks the the outside is welded and that weld is only inspected visually.  I think part of your problem is not going with a flux type of process which would handle welding through more contaminates then running solid wire with gas.
Reply:Not trying to confuse the issue here (leaking welds) but wouldn't it be better to look into spraying the inside of the tank with a "bedliner" type coating.This would isolate the RO water from the mild steel, as well as seal the welds.No matter how you "caulk" the welds, the contents will still attack the mild steel.When we produced potable water (US Army Corps of Engineers) via RO, our containment vessels were always either SS, porcelean/glass lined trailers, or rubber bladders.Syncro 250 DX Dynasty 200 DXMM 251 w/30A SG XMT 304 w/714 Feeder & Optima PulserHH187Dialarc 250 AC/DCHypertherm PM 1250Smith, Harris, Victor O/ASmith and Thermco Gas MixersAccess to a full fab shop with CNC Plasma, Water Jet, etc.
Reply:[QUOTE=SundownIII;410409]Not trying to confuse the issue here (leaking welds) but wouldn't it be better to look into spraying the inside of the tank with a "bedliner" type coating.This would isolate the RO water from the mild steel, as well as seal the welds.QUOTE]Now that's a hellova catch! Fix the structure and seal all of it up...Matt