The0retical's post and the fact that I had one needing to be done seemed to align. So I figured I would share how I usually do it. Others may have other methods. So do I, but this all this tank needs.
First installment:
This here is a fuel tank from an old Fiat
It is actually in really nice shape, but to see all of the issues it needs to be clean. The easy button for me is to send it to a local company who specializes in acid dipping cars. I give them $60-100 depending on the tank and get it back in a week or two like this
It is totally clean at this point. All rust, varnish, grease, oil and factory galvanizing is gone.
Now I realize I am very lucky to be able to have this done quickly and inexpensively so here is the GRM solution 
It is messy, kinda dangerous and inexpensive.
Go to Lowes Depot and purchase a gallon of muriatic acid
Plug all holes with rubber, hose clamps, tape... whatever. Fill your tank 3/4 full with HOT water. Pour in the acid. Block off the filler neck and rotate the tank every ten minutes until you are positive that every square millimeter has been exposed to the solution for 20 or more minutes.
Pour out the rusty acid solution into a drum or large bucket and neutralize for safety. I used baking soda
Marvel that the innards of your tank went from this
To this
Note: there is no way I would ever do this again. It was a huge, stinky, skin burning mess. Neutralizing the acid kept foaming it up, my patio is still stained from it 8 years later. But it was under $10 
If your tank is actually perforated and leaking fuel it will also be leaking the strong acid solution. It sucks, it is unnerving, your wife will hate you for it.
OK back to this Fiat tank today
now that it is clean we can see the issues
Tools for this step are
The liquid acid flux is only available via welding shops around here. It is critical. Flux pastes will not work. The solder is regular old lead free plumbing solder
Clean the area with a wire brush
heat with the propane torch, dab some flux with an acid brush to clean, apply the heat again and start flowing the solder over the area
File the high spots down (if you care to, totally unnecessary)
And continue to the next area.
It will be a mess of sticky burnt flux but that cleans off with a little solvent
When you have the tanks professionally dipped they have to drill a hole in them to ensure they get the air bubble out and the acid in. This one is 3/8" (they actually do this to the roofs of most cars too!)
Popped a 7/16" plug with the press punch
Soldered in place
Whole tank cleaned up
The reason I am soldering instead of welding is that fuel tanks are really, really thin. Some as thin as 24ga and to cut back far enough to get real metal to weld to I would most likely be remaking the entire bottom of this tank to fix that one corner. Brazing is another solution but the higher heat necessary will usually burn back the delicate lace of metal resulting in even larger gaps to try and seal.
The real star of this show is the tank sealant, which I ordered this morning. I usually use POR15 but I am really interested to try the Caswell product.
Next installment will be the cleaning and coating. Probably a week before the product arrives? Stay tuned, or don't. It is up to you.
