cockney steve wrote:AIUI, Aircraft aluminium skin -sheet is actually coated with a thin layer of alloy which abrasive is likely to wear-through.
Actually it's the other way around.
Not germain to the discussion, but for education purposes only.
The 'aluminium' used in aircraft construction is actually a mixture of about 95% aluminium and various other elements; often largely copper or zinc. These other elements are referred to as the 'alloying elements', and you end up with an 'aluminium alloy', also known as 'light alloy' or just 'alloy' in hangar slang (or even 'ali' - yuck and very confusing).
Aluminium alloy has much better strength than pure aluminium (which is pretty useless in this regard) and is capable of being 'heat treated' and hot or cold 'worked' (stretched, bashed, squashed, etc.) to improve its strength or hardness, as required.
The down-side is that aluminium alloy is more prone to corrosion than pure aluminium. So, the sheet material used in aircraft has a very thin layer of pure aluminium hot-rolled onto each face and this provides a good corrosion protection layer.
Such sheet material is 'aluminium-clad aluminium alloy sheet', usually shortened to 'alclad'.
OK, break time.