It looks like rust to me. I had a '63 Dart back in the 90's that kept clogging fuel filters with rust. I pulled the tank, drained it, mailed it off to a place in PA somewhere where they cut it in half, cleaned it thoroughly, welded it back together, and then coated it inside and out with a thick polymer coating. I don't remember too many specifics, but the outfit was called Moyers Fuel Tank Renu, and the work ran something like $200 around 1997.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.