Both my Stinson and Fairchild use welded aluminum tanks. I've helped redo a BT-13 center section. The original sealing method was zinc chromate paste. Messy to deal with, but sometimes more forgiving than polysulfide sealants.
On a fleet of Swearingen Merlins I had the misfortune to work on in the 80's the very thin fuel inspection covers were originally held on with polysulfide sealant. Because they were thin and polysulfide sticks like the proverbial baby stuff on a blanket the panels needed to be replaced every 100 hourly as they were destroyed in removal. Then along came an acceptable alternative to use a specific Loctite product (515? 525? - thick red stuff - form a gasket or some such name). The panels never leaked with that, it was easier to apply (no mixing and a far thinner paste) and best of all the panels almost fell off once the attachment screws were removed. Magic stuff.