Advice Needed Welding a Galvanized Tank

Assuming this is not a pressure tank you might be able to repair it (short term) with plumbers lead free solder, if you can get to the inside. Welding will probably just make the holes bigger as the rust is not going to be weldable. Welding will destroy the zink coating in the weld area and it will rust out that much quicker. Whittlin a stick into a plug and driving it in the hole with a hammer would probably be a better patch. (seriously) Then there is a chance you could coat the inside with a paint or fiberglass resin. If it is a pressure tank ... time for a new one. YMMV Glenn

Can you turn the tank upside down?
BoyntonStu

I would lap a patch on the outside and use E 6013 after grinding the zinc off where the welding bead will run. Keep in mind that you will burn off some of the galvanizing on the inside of the tank when you are welding on the outside so don't get carried away with big welds and high heats. With a bit of searching you might be able to get some heavy gauge galvanized material for your patch. Wire brush and paint the welded area with a zinc rich primer. Zinga and Galvacon come to mind. Zinc oxide fumes are a hazard but as long as you have an outside breeze and keep your head out of the plume there is no problem. Another way to go is to get a chunk of stainless plate and weld that on as your patch using E 309 electrode. You still will have to touch up the heat affected zone area with a zinc primer. Often when you have a leak on galvanized plate the material will be quite thin so your patch should be ridiculously oversize rather than end up chasing the Swiss cheese. Some people have mentioned patching with fiberglass or some filler/glue. If you grind the surface lightly, wipe clean with solvent something like this should work. I would look for a flexible patching material. Fiberglass resin tends to be brittle. A butyl mastic type roofing patch might even work. The big advantage would be that you don't damage the zinc coating with heat. If the tank is pressurized your choices are more limited. Randy

Do it cold. Apply a bolted plate of pre-galvanised steel sheet, coated with a suitable sealant paste. You can even do this on a tank still full of water.
You can weld galvanised sheet quite easily. It's unpleasant rather than toxic, so good ventilation is usually enough to suffice for you, along with working from the outside not the inside. However galvanised steel is also a poor welding substrate, which is the real reason for taking the zinc off with an angle grinder or hydrochloric acid beforehand. As with all plates on rusty steel, go back a _lot_ further than you expect to need - it's easy to make a bigger patch, it's hard to weld to a rusty edge, or to have the new edge pinhole through in no time afterwards.
My main reason to avoid welding is just that rusty steelwork is a pain to weld to. If you _can_ bolt a patch plate, it's probably easier and more long-lasting.
If you insist of welding, then MIG is the tool of choice.

If you intend to keep this tank in service for a long time, I would replace it. Chances are the holes in the bottom are just the tips of corrosion coming through to the outside. Even if you do manage to get the leaks stopped, (which may be difficult) you will soon be doing the same thing again in a dozen new places.
When you start to weld one of those "pinholes," you'll discover that the paper thin good metal is supported by a thick layer of rust, and it will melt back, making a large hole. The idea of welding patches over the holes could work, but you won't know whether there are other undetected thin places waiting to rust through.

Form a plate to match the shape of the tank at the hole. Lap this patch plate over the hole. Weld around the edges using 6010 electrode. 6010 is quite aggressive and easily able to burn through most galvanizing layers.
You could grind most of the zinc off and weld with 6011.
After welding, spray the whole area, inside and out, with cold galvanizing spray paint.

HVAC and appliance repair shops sell an epoxy bar (mix 2 parts) that is used on rusty galvanized drain pans and washing machine tubs. Only practical for a few holes but it doesn't damage the surrounding areas. As someone noted, if this is a stock tank, the rusting may be starting on the exterior, not the interior. When in doubt, follow Ernie's advice.
Ernie Leimkuhler wrote:

Thanks for all the ideas about how to repair my tank. First of all, I need to clarify a couple of things. The tank is for water storage for a well. I didn't think it was pressurized but now that I think of it I believe it is. I mean there is a pressure gauge on it. So, I guess it is pressurized. But I plan to drain it before doing the repair so I don't know if that is relevant or not.
It also doesn't actually have any visible holes in it yet but there are three areas that are seeping some water and there is visible rust in those areas. They are not very large rust spots though so I though if I get to them now I'll stop the process. Someone mentioned the possibility of more rust on the inside. I hadn't thought about that. But it only seems logical that if there are places where it is rusting through on the outside there may well be more of it on the inside that is not visible.
Like I said, I don't want to have to replace this tank if I don't have to because it will be expensive. If it is possible to fix it and get some more years use out of it I'd sure prefer that alternative.
Some of the ideas not requiring welding sound interesting and have made me rethink trying to do the repair by welding. On the other hand, Ernie's advice sounds good too. If I can just weld a patch over the bad spots with a 6010 electrode, paint it, and that does the job, that sounds like a good way to go. I'm going to have to think about this for a spell before I decide what to do. But not too long.
Hawke

Ever heard of freeze plugs?
They are pressed in brass alloy plugs that go into engine blocks, and if an engine freezes, they pop out before the freezing water can crack the block.
I bring that up, because, once I singlehandedly pulled the 283 cid motor out of my Chevy truck, and changed one. I was so proud. Then the next week, another popped. When I went back, the parts guy said, "No one ever changes one. Whatever shape that one was in, they are all in identical shape because they are all the same age."
DOH.
Well, I changed them all, having to pull the motor again.
Leaking tanks have provided similar experiences for me. By the time they leak, they are so thin, they don't weld very good, and one just blows holes in it when you hit a thin spot. Maybe you are lucky, and have one that only has a couple of rusted through spots, and can be fixed. Or maybe you have a case of metal cancer where the inevitable can only be prolonged a little longer.
If the tank can be visually inspected on the inside, do that, and try to see if you have a tank with a couple of holes, or if you need to make a fountain out of it by pressurizing it with water and enjoying the hundreds of squirting jets of water.
It's a crap shoot.
As per galvanized, a little grinding or electric wire brushing and if you don't do a lot of welding, no problem. If you do a lot of welding, use a respirator rated for that.
Just mho based on my experience.
Any time someone asks me to weld a few small holes in a tank, I run.
Steve

Replace it.
JTMcC.