Originally Posted by GearNut
He is not welding on the silver solder. He soldered a fuel fitting to the sheet metal using silver bearing solder and now plans on welding up the perimeter of said sheet metal to fabricate a fuel tank. He is concerned about the heat generated by welding which will travel through the sheet metal and may or may not be hot enough to mess up the silver solder job.
A rag soaked in cold water and wrapped around the fitting is the cure for this problem. But you have to remember that lots more people read this forum and not post at all. That is why we all should try and give as accurate information as we can. Joining metal is not to be taken lightly if done wrong the results can be costly on wallet and body parts.
After welding/brazing/silver soldering I would seal the tank with a sealer made to seal gastanks and use one that is not effected by Ethanol fuels.
It will prevent the tank from rusting and seal any pinhole leaks it may have.
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Learn to do it right or how to fix it later, quicker to do it right the first time.