Arc welding : How do I prevent these 'bubbles' from forming? How can I close them without grinding?

I'm self taught, was joining these diesel tanks to make water tanks, so the materials are the same thickness. As has happened many times before with other jobs these 'bubbles' form. It never really bothered me, but it turns out that water will leak out of the flux pool center. After much grinding my teeth and using the angle grinder the job is now complete and watertight. I would really like an experienced welder to give some advice.

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seandogue7 years ago
I would suggest that you grind those contaminated areas off and re-weld, since there's not all that much you can do aside from grinding off the bad material to ensure that the welds are contiguous and complete. Cause has already been well described.
caarntedd7 years ago
And chip the slag off and wire brush where the overlaps occur each time you stop or change rods.
Karroo Oakey (author)  caarntedd7 years ago
Thanks, I'm scrupulous about that - which is why this is baffling me so now.
NachoMahma7 years ago
.  It looks to me like you are not getting enough overlap when you finish the weld. Weld into your beginning weld a half-inch or so and then pul the rod out quickly. But I'm no expert. - try searching for "slag inclusion".
Karroo Oakey (author)  NachoMahma7 years ago
The slag inclusion as you say was in the middle of a bead. the photo is a little misleading as I'd already tried to close it up a number of times.
Nacho's right. That's a slag inclusion. The only way to seal it is to pick it all out Finish the pass by passing BACK into your weld by a 1/2" or so.

Spark up the next rod ON the previous pass, to drop down onto the end of the previous pass to avoid the inclusion.

And of course NEVER dip the rod in the weld pool !!

Looking at the picture, it looks like you haven't ground back the rust on the metal , and that is definitely affecting the quality of the interface on the upper plate.