Junkyard car's buildup (4)

I believe the answer is:

rust

(Other definitions for rust that I've seen before include "Reddish coating formed on iron through 11 across" , "Oxidised iron" , "Reddish-brown coating formed on iron by moisture" , "Oxidisation" , "Fungal disease of plants" .)

I've seen this clue in the Universal.

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