* Shine: If you're talking about the appearance of a metal surface, the antonym of rust would be shine.
* Protection: If you're talking about the process of rusting, the antonym could be protection, as in something that prevents rusting.
* Durability: If you're talking about the effect of rust on a metal object, the antonym could be durability.
For example:
* "The metal had a beautiful shine, unlike the rusty old tools in the shed."
* "A layer of paint provides protection against rust."
* "The stainless steel is renowned for its durability, unlike the iron that rusts easily."
So, the best antonym depends on what you're specifically referring to when you say "rust".