40% of panel receiving a metal coating (6)
I believe the answer is:
patina
'metal coating' is the definition.
(I've seen this in another clue)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for patina that I've seen before include "Brownish surface film on old bronze" , "Fine layer on the surface, laid down over time" , "Green gloss on old bronze" , "Sheen acquired from constant handling" , "Surface film formed by age" .)