Normal method used to attach pickup and sender rings to fuel tank

[Prarie dog]

Hello everyone,
First part of my question. Is the center area of rings on top of the fuel tank used to attach the plate on the sending unit and the pickup normally sealed to the section of sheet metal that comes up in the middle of the ring on top of the tank? In other words, are the surfaces, inner edge of the ring and the outside of vertical section of the tank that is the side of the opening, sealed with some material or is it just a narrow open gap? The ring seems to sit on a ledge that goes around around the opening. The ledge sets the height of the ring and supports it in the center. The stamping then drops down to allow room for the ends of the screws under the ring. At least on my tank!
What is the standard method used to attach the metal rings to the top of the fuel tank on an M38A1? I had thought welding would be the normal method but I have seen several images that look like they have been soldered in place. The surface of the seal around the rings is smooth and the material used is silver and softer then welding that I have seen. Since it is silver most likely not not brass brazing rod but maybe a different type of brazing rod? It does not have the "Tipped over stack of coins" look of a weld.
Thanks for information on this. I read though as many previous posts on tanks as I could find, I of course may have missed some, and saw info on the bottom drain plug repair. Welding, silver solder and brazing were mentioned but no info on the rings.
Going through older posts gave me a good insight into the issues with used tanks and resident fumes!
Thanks

[W. Winget]

Solder (made of lead) is the normal sealing mixture used by both radiator and fuel tank makers until aluminum and plastics came about.
The ring was soldered onto the tank.
If re-soldering, do not use flame unless the tank has been properly cleaned and torched as it is drained, or totally filled with water or non-explosive gas (like exhaust fumes)
Safest way is to take it to a radiator shop and let them do it.
JB weld is not really a perfect fix either.
V/R W Winget

[Prarie dog]

Thanks W Winget !
I had thought that is was a solder but was unsure. Would both the outer part of the ring and the inner diameter be attached / sealed to the tank? It is hard to tell in the pictures I have looked at. My guess was yes but I was unsure if the inner part needs to be left unattached for some reason.

I have a giant electric soldering iron used for doing stain glass leading, small spot car body filling and other tasks like sealing seams on sheet metal work to make it water tight. It can be a hazard by itself when it is hot if you are not careful! Think branding iron!

Based on some practice stuff I did using thin and thick steel sheet metal it seem like it would work well. No flame at all and a hot but localized heat source. Neutralizing and cleaning any flux out of the cavity below the ring will need some creative flushing with water and alcohol to dry things out.
My plan was to use CO2 to fill the tank when I am working. The tank has been cleaned / flushed several times and aired out. I think chunks of the local markets several pound block of dry ice sitting on a pad, to keep it from freezing the tank floor, inside the tank should provide a constantly expanding mass of CO2 gas that will over flow the volume of the tank and keep it filled and overflowing long enough to do the work. It will hopefully also not work as a heat sink like water might. So long as I stay higher then the top of the tank, and have ventilation to move the gas away I think I can avoid getting knocked out by the CO2 displacing the air around the tank as it comes out of the filler and ports. I guess we will find out.
Thanks again.

[W. Winget]

The outer edge is not that important, the sender has a gasket to seal the flanges, depending on the manufacturer, (and age) you may be able to slide a piece of cardstock into a gap. Just important to have the seal around the inner edge.
I have two of those soldering irons, something in the lightbulb dimming range when plugged in with about a 3/4 and 1" head, old cloth cord. Found them at various flea markets for about $5 each in like new condition.
That's the ticket if it can melt the solder. Make sure to not use the electronic solder.
And the rest you described well.
Good luck, and post some photos as you go, others can learn from it.
V/R W Winget