It's hard to tell from the photos, but the big flakes looked like varnish to me. If they're rust, they that's a different matter, but I've cleaned out tanks from what I thought was a lot of rust, and it was really a hardened layer of varnish that lacquer thinner slowly dissolved.
If you end up needing to line the tank, use Caswell's phenol novolac epoxy. It's the material that is used to line the big oil company storage tanks, and is resistant to all the junk in current gasoline.
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