Is rust countable or non-countable noun?

"Rust" can be both countable and uncountable, depending on how it's used:

Uncountable: When referring to the substance itself, it's uncountable:

* "There was rust on the old car."

* "He cleaned the rust off the tools."

Countable: When referring to individual instances or patches of rust, it's countable:

* "He found several rust spots on the metal."

* "The old fence had a few rusts on it."

It's important to consider the context to determine whether "rust" is countable or uncountable.