The Rover OEM lines are galvanised (zinc-coated) steel.
The white stuff is zinc oxide, which is the zinc corroding and protecting the steel.
(If even a tiny spec of steel is exposed to moisture - say from a stone chip - then an electrochemical cell is setup and the zinc starts corroding in preference to the steel).
Although its far too late for the cars now, best thing to do is remove any evidence of the zinc oxide (as well as any surface contamination, obviously) use zinc primer and then several coats of hard-wearing paint.
That way you've got paint protecting the zinc, which in turn is protecting the steel.
As an aside John, pure Copper yields (permanently deforms) at around 70MPa from memory, steels being 180-220MPa ish for typical mild steels, but can be all the way up to ~1000MPa for high-strength structural applications.
Hence, your estimation of needing twice as much effort to flare the steel is not very far wrong at all!
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