Tig welding stainless

You can use aluminum foil air duct tape. I use it a lot when TIG welding stainless. It will capture some of the argon from the weld that seeps through the gap, and use that to drive out the air.
The tape won't stay on for long. As the metal gets hot the glue will let go, but it should stay on long enough if you weld at a low enough amperage.
18 ga Stainless is around 0.047" which is going to take 30 amps to weld.

Thanks I will get some aluminiam tape. Also I need to tig a fitting on a stainless cylinder, what is the best way to prevent back oxidation? I can't access the inside of the cylinder and it's too big to fill with argon (100 liters).

I have had to weld fittings to brew tanks before. I took a pie tin and attached it to a broom handle. I had my assistant hold it against the inside of the tank from the top opening of the tank. I then flowed argon in from the fitting I was welding on. Worked great. A thin pie tin will flex to the inside curve of the tank. I have also had luck, when welding smaller fittings, using the cups from a silicone rubber cupcake baking tray. The cups take high heat, and are quite flexible. You could rig one up where you fold it up and slip it through the fitting on the tank so it unfolds inside the tank, then draw it back against the inside with a stick in the middle.
You learn lots of tricks when you do brewery repairs.

Are you alllowed to have any kind of residue on the back side? If your main objective is to protect the back of the weld then you might be able to use Solar B Staniless Steel Flux. The result will be a glassy residue that may be removed with a wire brush. If you do not have acesss to the back of your weld and can not brush off the residue this product is not for you. Backup tape is also produced for this purpose but also leaves a residue, its less than the Solar B but there is still residue. Argon purge, I would think after that. Hope this helps.
Rod

(100 liters).
I don't think you'd need to fill the tank much, just to purge the oxygen and maintain a small psi of argon. I don't know what volume of gas you'd need... maybe Ernie might know.