There's an aircraft fuel tank sealant known under various names such as Chem Seal & Pro Seal. It's a 2 part rubbery putty that comes in different colors & thick & thin consistancies. It's obviously made for use with aluminum & is used on aircraft for more than just sealing fuel tanks. I call it a flexible JB Weld 'cause it's pretty versatile. There are also sloshing sealers available. Here's a link to a few.
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