The3rd Earl Of Derby wrote:

The process involved dipping aluminium panels in caustic soda for a few
minutes whereby it gave the surface/facia a very hard resiliant
surface,almost none scratchable.

I think caustic soda would just dissolve the aluminium.

It's not anodising, is it?

Anodising increases the thickness of the surface oxide layer, which is
absorbent and can be dyed to pretty colours. That's also why they can't
get all the paint off underground trains when they've been defaced by
morons with spray cans.

Idiots' anodising guide here;
http://www.creative-chemistry.org.uk.../anodising.pdf

Caustic soda is involved, but only for initial cleaning. The anodising
solution uses sulphuric acid.

If it's not anodising, then just ignore that.