Paint treated a form of oxidation (6)

I believe the answer is:

patina

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'paint' is the definition.
(I know this)

I don't understand how the remainder of the clue works.

(Other definitions for patina that I've seen before include "Brownish surface film on old bronze" , "Surface shine" , "Surface film on ancient bronze" , "Sheen acquired from constant handling" , "Developing surface film; sheen" .)

I've seen this clue in The Mirror.

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