MIG - Air/Water Tight Weld

[Xobic]

I have recently become the new caretaker of a duck club. The previous caretaker was fired, then two months later was asked to come finish fabricating the duck blinds. Well long story short, three of the four float. After recovery of the blind and inspection, there were four places on the bottom where gaps were left in the steel pontoons. The gaps were up to half an inch by 12 inches. I welded up the gaps with the MIG welder and also welded an air valve to the top so I could check the pontoons for leaks. As it turns out, my welds are not air tight. Does anyone have any tips on airtight welds with a MIG welder? First time welder with MIG and it's become a bit frustrating.

[kcd616]

Your best and cheapest option is cut some pieces sheet metal and weld those over the leaks.
Because if you are MIG welding and not flux core or stick welding you should purge the inside of the weld to eliminate pin holes and porosity.
Also make sure you have removed all the paint and rust from the weld area.
Sincerely,
Kent

[arcs_n_sparks]

You need a controlled vent or plug if you are trying to weld an air/water tight chamber.

[Xobic]

The problem is the actual weld itself. I have an air stem to pressurize to test for leaks. That part works fine. I was more wondering about the pin holes in the bead. What causes pinholes and how can they be avoided? To fix the pinholes, do I need to grind it down and reweld, or try a bead on top of the holes or what? Now that I have them, what is the best way to get rid of them? Or should I just cut it out and start over? (I think that last one might be the frustration talking.)

[rlaggren]

You may have already allowed for this but you didn't say exactly so JIC:

I think the vent Arcs mentioned was something to leave _open_ while you welded so that the air pressure inside the closed sealed container doesn't rise and blow through your still molten welds. Leaving your vent you mentioned open while welding would certain take care of this.

Rufus

[daveco]

It sounds like these pontoons have already been in the water (?). If so, any moisture in the joint (or even inside the pontoon chamber) can certainly cause porosity.
Poor shielding gas coverage can also result in a spongy weld.
Are these being welded outdoors? Even a slight breeze can cause problems with shielding gas coverage. (the gas is on, yes?)
It is best to completely grind out a porous weld. Trying to weld over the top will just be an exercise in frustration
Like the others have mentioned, make sure the pontoon isn't being pressurized by the weld heat, the pressure will try to vent through the weld while you are welding, possibly causing porosity.

[tizzy]

Ive got a similar situation here. Outdoor wood stove, open system, not a pressurized boiler. It's (so I was told) A36. Near a weld that appeared to be undercut, it developed a leak from the water jacket into the fire. The company came out to fix it and they thought they were coming to fix a stainless unit. It appears they used a tig with stainless filler to attach a patch to the outside of the fire box. Two years later it began to leak out from the water jacket at the point where they cut a hole to get into the outside of the firebox. At that point I took the roof off to investigate and saw a nice hairline crack right down the center of the weld in the water jacket. Not thinking much of it at the time (bad on me) I fixed that leak. One year later its dripping back in the firebox. Now its time to cut it apart and replace the patch and stitch it back together.
I am going to cut a larger hole in the .125" water jacket to remove the failing weld. Next its time to grind off the patch on the outside of the .25" fire box. Attempt to locate the crack, clean it up, and weld it. Clean up the patch, put it back on. Using a larger piece I plan to lap over the hole in the water jacket.

Any other ideas?