Terneplate
The fuel tanks were normally made from a lead (& a small amount of tin) -coated steel sheet called Terneplate.
Very common sheet for fuel tanks and for applications where deep pressing was required, as the lead/tin slid nicely against the die formers, allowing deeper pressing without tearing the steel sheet. Also a very easy sheet to run solder along joints to seal them.
Alas, has not been available for many years.
It may be part of the problem with adherence of the magnet, combined with the thin sheet steel, both of which diminish the magnet's 'grip'.
Mike
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