If varnished, they won't go rusty (5)
I believe the answer is:
nails
I'm afraid I can't explain this answer.
Can you help me to learn more?
(Other definitions for nails that I've seen before include "Filed regularly" , "Part of body; hard as them" , "Fastens with spikes" , "Tacks - arrests" , "Horny plates of the digits" .)