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 can't explain the rest of the clue.
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" .)