In article ,
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.

Caustic soda etches aluminium, removing the protective
oxide film. It has to be handled carefully in this state
because it stains so easily. However a thin oxide film
does grow back fairly quickly. Professional anodising
grows a thicker film by electrolysis.

Radiospares used to do an aluminium labelling system
(was it Copyphot?) where the final stage was to immerse
the label in a hot solution of some magic salts that
chemically grew the protective oxide film.

--
Tony Williams.